Editing Lockheed P-38 Lightning (section)

==Design and development==
The Lockheed Corporation designed the P-38 in response to a February 1937 specification from the United States Army Air Corps (USAAC). Circular Proposal X-608 was a set of aircraft performance goals authored by [[First Lieutenant]]s [[Benjamin S. Kelsey]] and [[Gordon P. Saville]] for a twin-engined, high-altitude "interceptor" having "the tactical mission of interception and attack of hostile aircraft at high altitude."{{sfn|Bodie|2001|pp=16–17}} Forty years later, Kelsey explained that Saville and he drew up the specification using the word "interceptor" as a way to bypass the inflexible Army Air Corps requirement for pursuit aircraft to carry no more than {{convert|500|lb|kg|abbr=on}} of armament including ammunition, and to bypass the USAAC restriction of single-seat aircraft to one engine. Kelsey was looking for a minimum of {{convert|1000|lb|kg|abbr=on}} of armament.{{sfn|Bodie|2001|p=14}} Kelsey and Saville aimed to get a more capable fighter, better at dog fighting and at high-altitude combat. Specifications called for a maximum airspeed of at least {{convert|360|mph|km/h|abbr=on}} at altitude, and a climb to {{convert|20000|ft|m|abbr=on}} within six minutes,<ref name="Dave's">Hanson, Dave. [http://www.daveswarbirds.com/usplanes/aircraft/histp-38.htm "Lockheed P-38 Lightning."] ''Dave's Warbirds.'' Retrieved: 21 January 2007.</ref> the toughest set of specifications USAAC had ever presented. The unbuilt [[Vultee Aircraft|Vultee]] XP1015 design was offered to fill this requirement, but was not advanced enough to merit further investigation. A similar proposal for a single-engined fighter was issued at the same time, Circular Proposal X-609, in response to which the [[Bell P-39 Airacobra]] was designed.{{sfn|Bodie|2001|p=19}} Both proposals required liquid-cooled [[Allison V-1710]] engines with turbosuperchargers and gave extra points for [[tricycle undercarriage|tricycle landing gear]].

[[File:Lockheed P-38 Lightning fighter concept silhouettes.png|thumb|Preliminary Lockheed twin-engined P-38 fighter concepts]]
[[File:An armorer's assistant in a large western aircraft plant works on the installation of one of the machine guns in the... - NARA - 196367.jpg|thumb|P-38 armament, concentrated in the nose of the aircraft]]

Lockheed formed a secretive engineering team to implement the project apart from the main factory; this approach later became known as [[Skunk Works]].{{sfn|Bodie|2001|p=51}}<ref name="Wilson1969">{{Cite book |title=Current Biography Yearbook |date=1969 |publisher=H. W. Wilson Co. |page=199 |quote=At that time, Lockheed did not as yet have a formal engineering building, and so Johnson and his staff improvised a development plant using unoccupied corners in hangars and an old distillery. The results of this 'skunk works' approach was the legendary P-38 Lightning.}}</ref> The Lockheed design team, under the direction of [[Hall Hibbard]] and [[Kelly Johnson (engineer)|Clarence "Kelly" Johnson]], considered a range of twin-engined configurations, including both engines in a central fuselage with push–pull propellers.<ref>[http://p38assn.org/xpdrawings.htm "XP-38 Design Drawings: A diagram of the configurations considered for the prototype."] {{Webarchive|url=https://web.archive.org/web/20061218104035/http://p38assn.org/xpdrawings.htm |date=18 December 2006 }} ''P-38 National Association & Museum''. Retrieved: 21 January 2007.</ref>

The eventual configuration was rare in contemporary production fighter aircraft design, with the Dutch [[Fokker G.I]] heavy fighter, and the later [[Northrop P-61 Black Widow]] night fighter and Swedish [[SAAB 21]] having a similar [[planform (aeronautics)|planform]]. The Lockheed team chose twin booms to accommodate the tail assembly, engines, and turbosuperchargers, with a central nacelle for the pilot and armament. The XP-38 gondola mockup was designed to mount two .50-caliber (12.7&nbsp;mm) [[M2 Browning machine gun]]s with 200&nbsp;rounds per gun (rpg), two .30-caliber (7.62&nbsp;mm) Brownings with 500&nbsp;rpg, and a [[United States Army Ordnance Department]] prototype T1 23&nbsp;mm (.90 in) [[autocannon]] with a rotary magazine as a substitute for the nonexistent 25&nbsp;mm [[Hotchkiss et Cie|Hotchkiss]] aircraft autocannon specified by Kelsey and Saville.{{sfn|Bodie|2001|p=19}} In the prototype YP-38s, an Army Ordnance Department T9 37&nbsp;mm (1.46&nbsp;in) autocannon (later designated as the [[M4 autocannon|M4]] in production) with 15 rounds replaced the 23&nbsp;mm T1.{{sfn|Bodie|2001|p=44}}<ref name="Chinn 2022-01-07">{{cite book |last1=Chinn |first1=George |url=https://books.google.com/books?id=vI9KmcjzY2gC |title=The Machine Gun |chapter=37-mm Automatic Guns |volume=3 |location=Washington D.C., USA |publisher=United States Government Printing Office |date=1951 |page=31 |access-date=2022-01-07 |quote="Gun, Automatic 37-mm T9- The T2 gun was modified... until eventually a gun designated T9 was ready for test. In September 1939 this gun was mounted... in P-38 and P-39 fighter planes... which was standardized as the M4." }}</ref> The 15 rounds were in three, five-round clips, an unsatisfactory arrangement according to Kelsey, and the T9/M4 did not perform reliably in flight. Further armament experiments from March to June 1941 resulted in the P-38E combat configuration of four M2 Browning machine guns, and one [[Hispano-Suiza HS.404|Hispano]] 20&nbsp;mm (.79 in) autocannon with 150 rounds.{{sfn|Bodie|2001|p=80}}

Clustering all the armament in the nose was unusual in U.S. aircraft, which typically used wing-mounted guns with trajectories set up to crisscross at one or more points in a [[Gun harmonisation|convergence zone]]. The P-38 cannon used heavier 20&nbsp;mm rounds, creating a different trajectory, so it was inclined upward slightly more than the four machine guns such that the trajectories of the cannon rounds and .50-caliber bullets came together between 350 and 400 yards.<ref>AN 01-75-2 P-38 gunsight manual, Section IV, [https://qph.cf2.quoracdn.net/main-qimg-0ebf200ed7f00af35dc4c780838cc544 page 21]</ref> Nose-mounted guns did not suffer as much from having their useful ranges limited by pattern convergence, meaning that good pilots could shoot much farther. A Lightning could reliably hit targets at any range up to {{convert|1000|yd|m|abbr=on}}, whereas the wing guns of other fighters were optimized for a specific range.{{sfn|Coggins|2000||p=31}} The [[rate of fire]] was about 650 rounds per minute for the 20×110&nbsp;mm cannon round (130-gram shell) at a muzzle velocity of about {{convert|2850|ft/s|m/s|abbr=on}}, and for the .50-caliber machine guns (43-gram rounds), about 850&nbsp;rpm at {{convert|2900|ft/s|m/s|abbr=on}} velocity. Combined rate of fire was over 4,000&nbsp;rpm with roughly every sixth projectile a 20&nbsp;mm shell.<ref>{{cite magazine |url=https://books.google.com/books?id=_ykDAAAAMBAJ&pg=PA78 |pages=76–83, 186 |title=The Facts About Fighter-Plane Firepower |last=Grahame |first=Arthur |date=January 1944 |magazine=Popular Science}} Grahame says the Lightning shoots 168 rounds per second (combined cannon and MG), the weight of fire being 547 lb/minute (9.1 lb/second). The 20 mm cannon fires at 2850 ft/sec muzzle velocity, projectile weight 0.29 lb (130 grams), at 650 rpm (10.8 rps). The .50 caliber machine gun fires at 2900 ft/sec, weight of projectile 800 grains (51.8 grams), at 850 rev/min.</ref> The duration of sustained firing for the 20&nbsp;mm cannon was about 14 seconds, while the .50-caliber machine guns worked for 35 seconds if each magazine were <!-- subjunctive --> fully loaded with 500 rounds, or for 21 seconds if 300 rounds were loaded to save weight for long-distance flying.

The Lockheed design incorporated tricycle undercarriage and a bubble canopy, and featured two {{convert|1000|hp|kW|abbr=on}} turbosupercharged 12-cylinder Allison V-1710 engines fitted with [[counter-rotating propellers]] to eliminate the effect of engine [[torque]], with the turbochargers positioned behind the engines, the exhaust side of the units exposed along the dorsal surfaces of the booms.<ref name="Aviation Museum"/> Counter-rotation was achieved by the use of "handed" engines; the crankshafts of the engines turned in opposite directions, a relatively easy task for [[Allison V-1710#Versatility and reversibility of rotation|the V-1710 modular-design aircraft powerplant]].<ref>{{Cite web |year=1943 |title=Handbook of Operation and Maintenance-Allison V1710 type engines |url=https://stephentaylorhistorian.files.wordpress.com/2020/04/allison-v-1710-f-1943.pdf |publisher=Allison division, General Motors}}</ref>

The P-38 was the first American fighter to make extensive use of stainless steel and smooth, flush-riveted, butt-jointed aluminum skin panels.<ref>Loftin, L.K. Jr. 1985 [http://www.hq.nasa.gov/pao/History/SP-468/cover.htm "Quest for Performance: The Evolution of Modern Aircraft. NASA SP-468".] {{Webarchive|url=https://web.archive.org/web/20060613210139/http://www.hq.nasa.gov/pao/History/SP-468/cover.htm |date=13 June 2006 }} ''NASA Scientific and Technical Information Branch''. Washington. Retrieved: 22 April 2006.</ref>{{Page needed|date=October 2021}} It was also the first military airplane to fly faster than {{convert|400|mph|km/h|abbr=on}} in level flight.{{sfn|Thornborough|Davis|1988|p=8}}{{sfn|Bodie|2001|p=245}}

===XP-38 and YP-38 prototypes===
Lockheed won the competition on 23 June 1937 with its ''' Model 22''', and was contracted to build a prototype '''XP-38'''<ref name="O'Leary">O'Leary, Michael. [https://web.archive.org/web/20071012182446/http://findarticles.com/p/articles/mi_qa3901/is_200504/ai_n13498425/pg_1 "Conquering the Sky!"] ''Air Classics'', April 2005. Retrieved: 26 January 2007.</ref> for US$163,000, though Lockheed's own costs on the prototype would add up to $761,000.{{sfn|Bodie|2001|p=32}} Construction began in July 1938 in an old bourbon distillery purchased by Lockheed to house expanding operations. This secure and remote site was later identified by Johnson as the first of five Lockheed Skunk Works locations.{{sfn|Bodie|2001|p=51}}<ref name="Wilson1969"/><ref>{{Cite magazine |last=Kocivar |first=Ben |date=1964-10-06 |title=Collier Trophy |magazine=[[Look (American magazine)|Look]] |volume=28 |page=36 |quote=He calls his development plants 'skunk works'. There have been five of them&nbsp;– the first, an abandoned distillery. |number=20}}</ref> The XP-38 first flew on 27 January 1939 at the hands of Ben Kelsey.{{sfn|Bodie|2001|p=33}}{{refn|The 1939 edition of the ''German Aviation Manual ''already contained a detailed drawing and a close-up photograph of this prototype along with detailed information on the engines, and indicated that its maximum speed was supposed to be {{convert|640|–|680|km/h|mph|abbr=on}}. Dimensions, equipment, and weaponry were indicated as unknown.<ref>Schnitzler, R., G.W. Feuchter and R. Schulz, eds. ''Handbuch der Luftfahrt (Manual of Aviation)'' (in German). Munich: J.F. Lehmanns Verlag, 1939. pg386-7</ref>|group=Note}}

[[File:020903-o-9999b-059.jpg|thumb|One of 13 YP-38s constructed]]

Kelsey then proposed a speed dash to [[Wright-Patterson Air Force Base|Wright Field]] on 11 February 1939 to relocate the aircraft for further testing. [[Henry H. Arnold|General Henry "Hap" Arnold]], commander of the USAAC, approved of the record attempt and recommended a cross-country flight to New York. The flight set a speed record by flying from California to New York in seven hours and two minutes, not counting two refueling stops.<ref name="Aviation Museum">[http://www.aviation-history.com/lockheed/p38.html "Lockheed P-38 Lightning."] ''aviation-history.com.'' Retrieved: 21 January 2007.</ref> Kelsey flew conservatively for most of the way, working the engines gently, even throttling back during descent to remove the associated speed advantage. Bundled up against the cold, Arnold congratulated Kelsey at Wright Field during his final refueling stop, and said, "don't spare the horses" on the next leg.{{sfn|Bodie|2001|p=36}} After climbing out of Wright Field and reaching altitude, Kelsey pushed the XP-38 to {{convert|420|mph}}.{{sfn|Bodie|2001|p=40}} Nearing his destination, Kelsey was ordered by [[Mitchel Field]] tower ([[Hempstead (town), New York|Hempstead, New York]]) into a slow landing pattern behind other aircraft. Carburetor icing caused it to be brought down short of the Mitchel runway, and it was wrecked. On the basis of the record flight, though, the USAAC ordered 13 YP-38s on 27 April 1939 for US$134,284 (~${{Format price|{{Inflation|index=US-GDP|value=134284|start_year=1939}}}} in {{Inflation/year|US-GDP}}) each.{{sfn|Donald|1997|p=581}}{{sfn|Knaack|1988|p=3}} (The "Y" in "YP" was the USAAC's designation for service test, i.e. small numbers of early production aircraft, while the "X" in "XP" was for [[Experimental aircraft|experimental]].) Lockheed's chief [[test pilot]], [[Tony LeVier]], angrily characterized the accident as an unnecessary publicity stunt,{{sfn|Caidin|1983|p={{page needed|date=October 2021}}}} but according to Kelsey, the loss of the prototype, rather than hampering the program, sped the process by cutting short the initial test series. The success of the aircraft design contributed to Kelsey's promotion to captain in May 1939.{{sfn|Bodie|2001|p=51}}

[[File:Mechanized P-38 conveyor lines.jpg|thumb|Mechanized P-38 assembly lines in [[Burbank, California]]{{sfn|Parker|2013|pp=59, 75–76}}]]
Manufacture of YP-38s fell behind schedule, at least partly because of changes to meet the need for mass production, making them substantially different in construction from the prototype. Another factor was the sudden required expansion of Lockheed's facility in [[Burbank, California|Burbank]], taking it from a specialized civilian firm dealing with small orders to a large government defense contractor making [[Lockheed Ventura|Venturas]], [[Lockheed Ventura#PV-2 Harpoon|Harpoons]], [[Lockheed Model 18 Lodestar|Lodestars]], and [[Lockheed Hudson|Hudsons]], and designing the [[Lockheed Constellation|Constellation]] for [[Trans World Airlines|TWA]]. The first YP-38 was not completed until September 1940, with its maiden flight on 17 September.<ref>[http://p38assn.org/aboutp38.htm "About the P-38: Early Years."] ''P-38 National Association & Museum.'' Retrieved: 21 January 2007.</ref> The 13th and final YP-38 was delivered to the USAAC in June 1941; 12 aircraft were retained for flight testing and one for destructive stress testing. The YPs were substantially redesigned and differed greatly in detail from the hand-built XP-38. They were lighter and included changes in engine fit. The propeller rotation was reversed, with the blades spinning outward (away from the [[Cockpit (aviation)|cockpit]]) at the top of their arc, rather than inward as before. This improved the aircraft's stability as a gunnery platform.<ref name="Smithsonian">[http://collections.nasm.si.edu/code/emuseum.asp?style=single&currentrecord=1&page=search&profile=objects&searchdesc=A19600295000&quicksearch=A19600295000&newvalues=1&newstyle=expanded&newcurrentrecord=1 "Collections Database: Lockheed P-38J-10-LO Lightning."] National Air and Space Museum. Retrieved: 6 February 2009.</ref>

===High-speed compressibility problems===
[[File:Lockheed P-38G cockpit looking in from left wing 061019-F-1234P-004.jpg|thumb|left|upright|The P-38 was flown with a yoke, rather than the more-usual stick.]]

Test flights revealed problems initially believed to be tail [[Wing flutter|flutter]]. During high-speed flight approaching Mach&nbsp;0.68, especially during dives, the aircraft's tail would begin to shake violently, and the nose would tuck under (see [[Mach tuck]]), steepening the dive. Once caught in this dive, the fighter would enter a high-speed compressibility stall and the controls would lock up, leaving the pilot no option but to bail out (if possible) or remain with the aircraft until it got down to denser air, where he might have a chance to pull out. During a test flight in May 1941, USAAC Major Signa Gilkey managed to stay with a YP-38 in a compressibility lockup, riding it out until he recovered gradually using [[Trim tab|elevator trim]].<ref name="Aviation Museum"/> Lockheed engineers were very concerned by this limitation, but first had to concentrate on filling the current order of aircraft. [[1941 in aviation#June|In late June 1941]], the Army Air Corps was renamed the [[U.S. Army Air Forces]] (USAAF), and 65 Lightnings were finished for the service by September 1941, with more on the way for the USAAF, the [[Royal Air Force]] (RAF), and the Free French Air Force operating from England.

By November 1941, many of the initial assembly-line challenges had been met, which freed up time for the engineering team to tackle the problem of frozen controls in a dive. Lockheed had a few ideas for tests that would help them find an answer. The first solution tried was the fitting of spring-loaded servo tabs on the elevator trailing edge designed to aid the pilot when control yoke forces rose over {{convert|30|lb-f|N}}, as would be expected in a high-speed dive. At that point, the tabs would begin to multiply the effort of the pilot's actions. Expert test pilot Ralph Virden was given a specific high-altitude test sequence to follow and was told to restrict his speed and fast maneuvering in denser air at low altitudes, since the new mechanism could exert tremendous leverage under those conditions. A note was taped to the instrument panel of the test craft underscoring this instruction. On 4 November 1941, Virden climbed into YP-38 #1 and completed the test sequence successfully, but 15 minutes later, was seen in a steep dive followed by a high-G pullout. The tail unit of the aircraft failed at about {{convert|3500|ft|m|sigfig=1|abbr=on}} during the high-speed dive recovery; Virden was killed in the subsequent crash. The Lockheed design office was justifiably upset, but their design engineers could only conclude that servo tabs were ''not'' the solution for loss of control in a dive. Lockheed still had to find the problem; the USAAF personnel were sure it was flutter, and ordered Lockheed to look more closely at the tail.

In 1941, flutter was a familiar engineering problem related to a too-flexible tail, but the P-38's [[empennage]] was completely skinned in aluminum rather than fabric and was quite rigid. At no time did the P-38 suffer from true flutter.{{sfn|Bodie|2001|p=58}} To prove a point, one elevator and its vertical stabilizers were skinned with metal 63% thicker than standard, but the increase in rigidity made no difference in vibration. Army [[Lieutenant Colonel]] Kenneth B. Wolfe (head of Army Production Engineering) asked Lockheed to try external mass balances above and below the elevator, although the P-38 already had large mass balances elegantly placed within each vertical stabilizer. Various configurations of external mass balances were equipped, and dangerously steep test flights were flown to document their performance. Explaining to Wolfe in Report No. 2414, Kelly Johnson wrote, "the violence of the vibration was unchanged and the diving tendency was naturally the same for all conditions." The external mass balances did not help at all. Nonetheless, at Wolfe's insistence, the additional external balances were a feature of every P-38 built from then on.{{sfn|Bodie|2001|p=57}}

[[File:Compressibility010.png|thumb|The P-38 pilot training manual compressibility chart shows speed limit vs. altitude.]]
Johnson said in his autobiography{{sfn|Johnson|Smith|1985|p=74}} that he pleaded with [[National Advisory Committee for Aeronautics]] to do model tests in its wind tunnel. They already had experience of models thrashing around violently at speeds approaching those requested and did not want to risk damaging their tunnel. Gen. Arnold, head of USAAF, ordered them to run the tests, which were done up to Mach 0.74.<ref>Erikson, Albert L. "Wind-Tunnel Investigation of Devices for Improving The Diving Characteristics of Airplanes." ''NACA MR No. 3F12, Summary''.</ref> The P-38's dive problem was revealed to be the [[center of pressure (fluid mechanics)|center of pressure]] moving back toward the tail when in high-speed airflow. The solution was to change the geometry of the wing's lower surface when diving to keep lift within bounds of the top of the wing. In February 1943, quick-acting dive flaps were tried and proven by Lockheed test pilots. The dive flaps were installed outboard of the engine nacelles, and in action, they extended downward 35° in 1.5 seconds. The flaps did not act as a speed brake; they affected the pressure distribution in a way that retained the wing's lift.{{sfn|Bodie|2001|pp=174–175}}

Late in 1943, a few hundred dive flap field-modification kits were assembled to give North African, European, and Pacific P-38s a chance to withstand compressibility and expand their combat tactics. The kits did not always reach their destination. In March 1944, 200 dive flap kits intended for the [[European Theater of Operations]] (ETO) P-38Js were destroyed in a mistaken [[identification friend or foe|identification]] incident in which an RAF fighter shot down the [[Douglas C-54 Skymaster]] (mistaken for a German [[Focke-Wulf Fw 200]]) taking the shipment to England. Back in Burbank, P-38Js coming off the assembly line in spring 1944 were towed out to the ramp and modified in the open air. The flaps were finally incorporated into the production line in June 1944 on the last 210 P-38Js. Despite testing having proved the dive flaps effective in improving tactical maneuvers, a 14-month delay in production limited their implementation, with only the final half of all Lightnings built having the dive flaps installed as an assembly-line sequence.{{sfn|Ethell|1984|p=14}}

Johnson later recalled:
{{Blockquote|I broke an ulcer over compressibility on the P-38 because we flew into a speed range where no one had ever been before, and we had difficulty convincing people that it wasn't the funny-looking airplane itself, but a fundamental physical problem. We found out what happened when the Lightning shed its tail and we worked during the whole war to get 15 more kn [28&nbsp;km/h] of speed out of the P-38. We saw compressibility as a brick wall for a long time. Then we learned how to get through it.<ref>Goebel, Greg. {{usurped|1=[https://web.archive.org/web/20020223121418/http://www.vectorsite.net/avp38.html "The Lockheed P-38 Lightning."]}} ''vectorsite.net'', Version 1.3. Retrieved: 21 January 2007.</ref>}}

[[Buffeting]] was another early aerodynamic problem. Distinguishing it from compressibility was difficult, as both were reported by test pilots as "tail shake". Buffeting came about from airflow disturbances ahead of the tail; the airplane would shake at high speed. Leading-edge wing slots were tried, as were combinations of [[fillet (mechanics)|filleting]] between the wing, cockpit, and engine nacelles. Air-tunnel test number 15 solved the buffeting completely and its fillet solution was fitted to every subsequent P-38 airframe. Fillet kits were sent out to every squadron flying Lightnings. The problem was traced to a 40% increase in air speed at the wing-fuselage junction where the thickness/chord ratio was highest. An airspeed of {{convert|500|mph|km/h|abbr=on}} at {{convert|25000|ft|m|abbr=on}} could push airflow at the wing-fuselage junction close to the speed of sound. Filleting solved the buffeting problem for the P-38E and later models.{{sfn|Bodie|2001|p=58}}

[[File:Lockheed P-38 fighter plane engines slnsw.jpg|thumb|upright|Airfield crew working on Lockheed P-38 fighter plane engines, ''circa'' 1944]]
Another issue with the P-38 arose from its unique design feature of outwardly rotating (at the "tops" of the propeller arcs) counter-rotating propellers. Losing one of two engines in any twin-engined, non-[[centerline thrust]] aircraft on takeoff creates sudden drag, yawing the nose toward the dead engine and rolling the wingtip down on the side of the dead engine. Normal training in flying twin-engined aircraft when losing an engine on takeoff is to push the remaining engine to full throttle to maintain airspeed; if a pilot did that in the P-38, regardless of which engine had failed, the resulting engine torque and [[p-factor]] force produced a sudden, uncontrollable yawing roll, and the aircraft would flip over and crash. Eventually, procedures were taught to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the failed engine, and then increasing power gradually until the aircraft was in stable flight. Single-engined takeoffs were possible, though not with a full fuel and ammunition load.{{sfn|Bodie|2001|p=210}}

The engines were unusually quiet because the exhausts were [[muffler|muffled]] by the [[General Electric]] turbosuperchargers on the twin Allison V-12s.{{sfn|Kaplan|Saunders|1991|p=56}} Early problems with cockpit temperature regulation occurred; pilots were often too hot in the tropical sun, as the canopy could not be fully opened without severe buffeting, and were often too cold in Northern Europe and at high altitude, as the distance of the engines from the cockpit prevented easy heat transfer. Later variants received modifications (such as electrically heated flight suits) to solve these problems.{{citation needed|date=September 2023}}

On 20 September 1939, before the YP-38s had been built and flight tested, the USAAC ordered 66 initial-production P-38 Lightnings, 30 of which were delivered to the (renamed) USAAF in mid-1941, but not all these aircraft were armed. The unarmed aircraft were subsequently fitted with four .50 in (12.7&nbsp;mm) machine guns (instead of the two .50 in/12.7&nbsp;mm and two .30 in/7.62&nbsp;mm of their predecessors) and a 37&nbsp;mm (1.46&nbsp;in) cannon. They also had armored glass, cockpit armor, and [[Fluorescence|fluorescent]] instrument lighting.<ref name="Baugher P-38">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38 "Lockheed P-38 Lightning."] {{Webarchive|url=https://web.archive.org/web/20120526012706/http://www.joebaugher.com/usaf_fighters/p38 |date=26 May 2012 }} ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref> One was completed with a pressurized cabin on an experimental basis and designated '''XP-38A'''.<ref name="Baugher XP-38A">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_1.html "Lockheed XP-38A Lightning."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref> Due to reports the USAAF was receiving from Europe, the remaining 36 in the batch were upgraded with small improvements such as [[self-sealing fuel tank]]s and enhanced armor protection to make them combat-capable. The USAAF specified that these 36 aircraft were to be designated '''P-38D'''. As a result, no P-38Bs or P-38Cs were designated. The P-38D's main role was to work out bugs and give the USAAF experience with handling the type.<ref name="Baugher P-38D">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_6.html "Lockheed P-38D Lightning."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 13 June 1999. Retrieved: 29 January 2007.</ref>

[[File:P-38 Lightning at sunset.jpg|thumb|P-38 rear view]]
In March 1940, the French and British, through the [[British Purchasing Commission|Anglo-French Purchasing Committee]], ordered 667 P-38s for US$100M,{{sfn|Bodie|2001|p=46}} designated '''Model 322F''' for the French and '''Model 322B''' for the British. The aircraft was a variant of the P-38E. The overseas Allies wished for complete commonality of Allison engines with the large numbers of [[Curtiss P-40 Warhawk|Curtiss P-40 Tomahawks]] both nations had on order, so they ordered the aircraft fitted with two right-handed engines (not counter-rotating) without turbosuperchargers.{{sfn|Bodie|2001|pp=45, 47}}{{refn|Turbosuperchargers were not secret nor restricted by the United States government. Related designs were known from French and Swiss firms. France and the UK did not want turbosuperchargers; they had never employed them and they knew the American ones were in short supply and did not want delivery delayed<ref name="Baugher Lightning I"/>|group=Note}} Performance was supposed to be {{convert|400|mph|km/h|abbr=on}} at {{convert|16900|ft|m|abbr=on}}.<ref name="Baugher Lightning I">Baugher, Joe. [http://www.joebaugher.com/usaf_fighters/p38_7.html "Lightning I for RAF."] ''Joe Baugher's Encyclopedia of American Military Aircraft'', 2 December 2002. Retrieved: 29 January 2007.</ref> After the [[fall of France]] in June 1940, the British took over the entire order and gave the aircraft the [[British military aircraft designation systems|service name]] "Lightning". By June 1941, the War Ministry had cause to reconsider their earlier aircraft specifications based on experience gathered in the [[Battle of Britain]] and [[the Blitz]].{{sfn|Bodie|2001|p=60}} British displeasure with the Lockheed order came to the fore in July, and on 5 August 1941, they modified the contract such that 143 aircraft would be delivered as previously ordered, to be known as "Lightning (Mark) I", and 524 would be upgraded to US-standard P-38E specifications with a top speed of {{convert|415|mph|km/h|abbr=on}} at {{convert|20000|ft|m|abbr=on}} guaranteed, to be called "Lightning II", for British service.{{sfn|Bodie|2001|p=60}} Later that summer, an RAF test pilot reported back from Burbank with a poor assessment of the "tail flutter" situation, and the British cancelled all but three of the 143 Lightning Is.{{sfn|Bodie|2001|p=60}} As a loss around US$15M was involved, Lockheed reviewed their contracts and decided to hold the British to the original order. Negotiations grew bitter and stalled.{{sfn|Bodie|2001|p=60}} Everything changed after Japanese [[attack on Pearl Harbor]] on 7 December 1941, after which the United States government seized some 40 of the Model 322s for [[West Coast of the United States|West Coast]] defense;{{sfn|Bodie|2001|p=63}} subsequently, all British Lightnings were delivered to the USAAF starting in January 1942. The USAAF lent the RAF three of the aircraft, which were delivered by sea in March 1942{{sfn|Bodie|2001|p=61}} and were test flown no earlier than May{{sfn|Bodie|2001|p=64}} at [[Cunliffe-Owen Aircraft]] Swaythling, the [[Aeroplane and Armament Experimental Establishment]] (A&AEE) and the [[Royal Aircraft Establishment]].{{sfn|Bodie|2001|p=60}} The A&AEE example was unarmed, lacked turbochargers, and restricted to {{convert|300|mph|km/h|abbr=on}}, though the undercarriage was praised and flight on one engine described as comfortable.{{sfn|Mason|2010|pp=204–205}} These three were subsequently returned to the USAAF, one in December 1942 and the others in July 1943.{{sfn|Bodie|2001|p=61}} Of the remaining 140 Lightning Is, 19 were not modified and were designated by the USAAF as '''RP-322-I''' ('R' for 'Restricted', because noncounter-rotating propellers were considered more dangerous on takeoff), while 121 were converted to counter-rotating V-1710F-2 engines without turbosuperchargers and designated '''P-322-II'''. All 121 were used as [[Trainer (aircraft)|advanced trainers]]; a few were still serving that role in 1945.{{sfn|Bodie|2001|p=64}} A few RP-322s were later used as test-modification platforms such as for smoke-laying canisters. The RP-322 was a fairly fast aircraft below {{convert|16000|ft|m|abbr=on}} and well-behaved as a trainer.{{sfn|Bodie|2001|p=64}}{{refn|Some of the fastest postwar racing P-38s were virtually identical in layout to the P-322-II.|group=Note}}

Many of the British order of 524 Lightning IIs were fitted with stronger F-10 Allison engines as they became available, and all were given wing pylons for fuel tanks or bombs. The upgraded aircraft were deployed to the Pacific as USAAC F-5A reconnaissance or P-38G fighter models, the latter used with great effect in the [[Operation Vengeance|operation that shot down Admiral Yamamoto]] in April 1943. Robert Petit's G model named ''Miss Virginia'' was on that mission, borrowed by [[Rex T. Barber|Rex Barber]], who was later credited with the kill. Petit had already used ''Miss Virginia'' to defeat two [[Nakajima A6M2-N]] "Rufe" floatplanes in February and to heavily damage a [[No.1-class auxiliary submarine chaser|Japanese submarine chaser]] in March, which he mistakenly claimed as a destroyer sunk. Murray "Jim" Shubin used a less-powerful F model he named ''Oriole'' to down five confirmed and possibly six Zeros over Guadalcanal in June 1943 to become ace in a day.{{sfn|Bodie|2001|pp=111–116}}

The British name was retained over Lockheed's original name ''[[Atalanta]]'', the swift-running Greek goddess, following the company tradition of using mythological and celestial figures.{{sfn|Yenne|1987|p=60}}

===Range extension===
The strategic-bombing proponents within the USAAF, nicknamed the [[Bomber Mafia]] by their ideological opponents, had established in the early 1930s a policy against research to create long-range fighters, which they thought would not be practical; this kind of research was not to compete for bomber resources. Aircraft manufacturers understood that they would not be rewarded if they installed subsystems on their fighters to enable them to carry [[drop tank]]s to provide more fuel for extended range. Lieutenant Kelsey, acting against this policy, risked his career in late 1941 when he convinced Lockheed to incorporate such subsystems in the P-38E model, without putting his request in writing.   Kelsey possibly was responding to Colonel [[George William Goddard]]'s observation that the US sorely needed a high-speed, long-range [[photo reconnaissance]] plane. Along with a [[change order]] specifying some P-38Es be produced with guns replaced by photoreconnaissance cameras, to be designated the F-4-1-LO, Lockheed began working out the problems of drop-tank design and incorporation. After the attack on Pearl Harbor, eventually about 100 P-38Es were sent to a modification center near [[Dallas, Texas]], or to the new Lockheed assembly plant B-6 (today the [[Burbank Airport]]), to be fitted with four K-17 [[aerial photography]] cameras. All of these aircraft were also modified to be able to carry drop tanks. P-38Fs were modified, as well. Every Lightning from the P-38G onward was capable of being fitted with drop tanks straight off the assembly line.{{sfn|Bodie|2001|pp=89–91}}

In March 1942, General Arnold made an off-hand comment that the US could avoid the [[Battle of the Atlantic|German U-boat menace]] by flying fighters to the UK rather than packing them onto ships. [[Franklin D. Roosevelt|President Roosevelt]] pressed the point, emphasizing his interest in the solution. Arnold was likely aware of the flying radius extension work being done on the P-38, which by this time had seen success with small drop tanks in the range of {{convert|150|to|165|gal|L|abbr=on}}, the difference in capacity being the result of subcontractor production variation. Arnold ordered further tests with larger drop tanks in the range of {{convert|300|to|310|gal|L|abbr=on}}; the results were reported by Kelsey as providing the P-38 with a {{convert|2500|mi|km|adj=on}} ferrying range.{{sfn|Bodie|2001|pp=89–91}} Because of available supply, the smaller drop tanks were used to fly Lightnings to the UK, the plan called [[Operation Bolero]].

Led by two [[Boeing B-17 Flying Fortress]]es, the first seven P-38s, each carrying two small drop tanks, left [[Presque Isle International Airport|Presque Isle Army Air Field]] in Maine on 23 June 1942 for [[RAF Heathfield]] in Scotland. Their first refueling stop was made in far northeast Canada at [[CFB Goose Bay|Goose Bay]]. The second stop was a rough airstrip in Greenland called [[Narsarsuaq Air Base|Bluie West One]], and the third refueling stop was in [[Iceland]] at [[Naval Air Station Keflavik|Keflavik]]. Other P-38s followed this route, with some lost in mishaps, usually due to poor weather, low visibility, radio difficulties, and navigational errors. Nearly 200 of the P-38Fs (and a few modified Es) were successfully flown across the Atlantic in July–August 1942, making the P-38 the first USAAF fighter to reach Britain and the first fighter ever to be delivered across the Atlantic under its own power.{{sfn|McFarland|Newton|2006|p=103}} Kelsey himself piloted one of the Lightnings, landing in Scotland on 25 July.{{sfn|Bodie|2001|pp=101–102}}