Editing De Havilland Mosquito (section)

==Development==
[[File:DH88 Comet G-ACSS Grosvenor House (6741273695).jpg|thumb|Construction concepts pioneered in the [[De Havilland DH.88|DH.88 Comet]] were later used in the Mosquito.]]
By the early to mid-1930s, [[de Havilland]] had built a reputation for innovative high-speed aircraft with the [[de Havilland DH.88|DH.88 Comet]] racer. Later, the [[de Havilland Albatross|DH.91 Albatross]] airliner pioneered the composite wood construction used for the Mosquito. The 22-passenger Albatross could cruise at {{cvt|210|mph|km/h}} at {{cvt|11000|ft|m}},  faster than the [[Handley Page H.P.42]] and other biplanes it was replacing.<ref>Stroud 1990, p. 369.</ref>{{Efn|Part 2 of article is in the July 1990 volume.}} The wooden [[monocoque]] construction not only saved weight and compensated for the low power of the [[de Havilland Gipsy Twelve]] engines used by this aircraft, but also simplified production and reduced construction time.<ref name="Bowman 2005, p. 9.">Bowman 2005, p. 9.</ref>

===Air Ministry bomber requirements and concepts===
On 8 September 1936, the British Air Ministry issued [[list of Air Ministry specifications|Specification]] P.13/36, which called for a twin-engined medium bomber capable of carrying a bomb load of {{cvt|3,000|lb|kg}} for {{cvt|3000|mi|km}} with a maximum speed of {{cvt|275|mph|km/h|round=5}} at {{cvt|15000|ft|m}}; a maximum bomb load of {{cvt|8000|lb|kg}} carried over shorter ranges was also required.<ref name="Sharb30">Sharp and Bowyer 1971, p. 30.</ref> Aviation firms entered heavy designs with new high-powered engines and multiple defensive turrets, leading to the production of the [[Avro Manchester]] and [[Handley Page Halifax]].<ref name="Sharb31">Sharp and Bowyer 1971, p. 31.</ref>

In May 1937, as a comparison to P.13/36, [[George Volkert]], the chief designer of [[Handley Page]], put forward the concept of a fast, unarmed bomber. In 20 pages, Volkert planned an aerodynamically clean, medium bomber to carry {{cvt|3000|lb|kg}} of bombs at a cruising speed of {{cvt|300|mph|km/h|round=5}}. Support existed in the RAF and Air Ministry; Captain R. N. Liptrot, Research Director Aircraft 3, appraised Volkert's design, calculating that its top speed would exceed that of the new [[Supermarine Spitfire]], but counter-arguments held that although such a design had merit, it would not necessarily be faster than enemy fighters for long.<ref>Buttler 2004, pp. 76–77.</ref> The ministry was also considering using non-[[strategic materials]] for aircraft production, which, in 1938, had led to specification B.9/38 and the [[Armstrong Whitworth Albemarle]] medium bomber, largely constructed from spruce and plywood attached to a steel-tube frame. The idea of a small, fast bomber gained support at a much earlier stage than is sometimes acknowledged, though the Air Ministry likely envisaged it using light alloy components.<ref>Buttler 2004, pp. 74, 77.</ref>

===Inception of the de Havilland fast bomber===
[[File:Albatross 1938 prototype.jpg|thumb|One of de Havilland's proposals was to adapt the [[de Havilland Albatross]] design to create a fast bomber.]]
Based on his experience with the Albatross, Geoffrey de Havilland believed that a bomber with a good aerodynamic design and smooth, minimal skin area, would exceed the P.13/36 specification.<ref name="Birtles 2017, ch. 2">{{Harvnb | Birtles | 2017 | at = ch. 2}}.</ref> Furthermore, adapting the Albatross principles could save time. In April 1938, performance estimates were produced for a twin [[Rolls-Royce Merlin]]-powered DH.91, with the [[Bristol Hercules]] ([[radial engine]]) and [[Napier Sabre]] ([[H-engine]]) as alternatives.<ref name= "Sharb30" />  On 7 July 1938, de Havilland wrote to [[Air Marshal]] [[Wilfrid Freeman]], the Air Council's member for Research and Development, discussing the specification and arguing that in war, shortages of aluminium and steel would occur, but supplies of wood-based products were "adequate."{{Efn|The situation had changed by October 1941, Connor cites a message<ref name="Connor-Madison, pp. 2-3."/> that Britain was desperately short: – "One of four airscrew factories has run out of imported veneers… veneers for air frame plywood would enable… construction to continue till December only… direct contact between Ministry of Supply experts and U.S.A. suppliers of aircraft veneers will be necessary for next three months. Otherwise essential war production… will be further curtailed." Wood construction used a different sector of workforce, and at the time, dH's experience was almost exclusively in the use of timber.<ref>Cypher dated 2 October 1941: – To – British Supply Council in North America. From – Supply Committee.  LONUS 13, National Archives. London.</ref>}} Although inferior in [[Tension (physics)|tension]], the [[Specific strength|strength-to-weight ratio]] of wood is equal to or better than light alloys or steel, hence this approach was feasible.<ref name="Sharb30"/><ref name= "Birtles 2017, ch. 2" /> [[Max Aitken, 1st Baron Beaverbrook|Lord Beaverbrook]], Minister of Aircraft Production, nicknamed it "Freeman's Folly", alluding to Air Chief Marshal Sir [[Wilfrid Freeman]], who defended [[Geoffrey de Havilland]] and his design concept against orders to scrap the project.{{Sfn | Birtles | 2017 | at = ch. 3}}

A follow-up letter to Freeman on 27 July said that the P.13/36 specification could not be met by a twin Merlin-powered aircraft and either the top speed or load capacity would be compromised, depending on which was paramount. For example, a larger, slower, turret-armed aircraft would have a range of {{cvt|1500|mi|km}} carrying a 4,000&nbsp;lb bomb load, with a maximum of {{cvt|260|mph|km/h}} at {{cvt |19000|ft|m}}, and a cruising speed of {{cvt|230|mph|km/h}} at {{cvt|18000|ft|m}}. De Havilland believed that a compromise, including eliminating surplus equipment, would improve matters.<ref name="Sharb30"/> On 4 October 1938, de Havilland projected the performance of another design based on the Albatross, powered by two Merlin&nbsp;Xs, with a three-man crew and six or eight forward-firing guns, plus one or two manually operated guns and a tail turret. Based on a total loaded weight of {{cvt|19000|lb|kg|abbr=on}}, it would have a top speed of {{cvt|300|mph|km/h|abbr=on}} and cruising speed of {{cvt |268|mph|km/h|abbr=on}} at {{cvt|22500|ft|m|abbr=on}}.<ref name="Sharb31" />

Still believing this could be improved, and after examining more concepts based on the Albatross and the new all-metal [[de Havilland Flamingo|DH.95 Flamingo]], de Havilland settled on designing a new aircraft that would be aerodynamically clean, wooden, and powered by the Merlin, which offered substantial future development.<ref name="Sharb31"/> The new design would be faster than foreseeable enemy [[fighter aircraft]], and could dispense with defensive armament which would slow it and make interception or losses to antiaircraft guns more likely. Instead, high speed and good manoeuvrability would make evading fighters and ground fire easier.<ref name="Sharb31" /> The lack of turrets simplified production, reduced drag, and reduced production time, with a delivery rate far in advance of competing designs. Without armament, the crew could be reduced to a pilot and navigator. Whereas contemporary RAF design philosophy favoured well-armed heavy bombers, this mode of design was more akin to the German philosophy of the ''[[Schnellbomber]]''.<ref name= "Batchelor and Lowe 2008, p. 5.">Batchelor and Lowe 2008, p. 5.</ref>  At a meeting in early October 1938 with Geoffrey de Havilland and Charles Walker (de Havilland's chief engineer), the Air Ministry showed little interest, and instead asked de Havilland to build wings for other bombers as a subcontractor.<ref name="Batchelor and Lowe 2008, p. 6.">Batchelor and Lowe 2008, p. 6.</ref>

By September 1939, de Havilland had produced preliminary estimates for single- and twin-engined variations of light-bomber designs using different engines, speculating on the effects of defensive armament on their designs.<ref name="Buttler, p. 78">Buttler 2004, p. 78.</ref> One design, completed on 6 September, was for an aircraft powered by a single {{cvt|2000|hp|kW|abbr=on}}  Napier Sabre, with a wingspan of {{cvt|47|ft|m}} and capable of carrying a {{cvt|1000|lb|kg|adj=on}} bomb load {{cvt|1500|mi|km}}. On 20 September, in another letter to Wilfrid Freeman, de Havilland wrote "…we believe that we could produce a twin-engine bomber which would have a performance so outstanding that little defensive equipment would be needed."<ref name= "Buttler, p. 78"/> By 4 October, work had progressed to a twin-engined light bomber with a wingspan of {{cvt |51|ft|m|abbr=on}} and powered by Merlin or Griffon engines, the Merlin favoured because of availability.<ref name="Buttler, p. 78"/> On 7 October 1939, a month into the war, the nucleus of a design team under [[Ronald Eric Bishop|Eric Bishop]] moved to the security and secrecy of [[De Havilland Aircraft Heritage Centre#Salisbury Hall|Salisbury Hall]] to work on what was later known as the DH.98.<ref name="Sharb34">Sharp and Bowyer 1971, p. 34.</ref>{{Efn|Bishop had worked on the DH.88 Comet and the Albatross under de Havilland's then-chief designer A. E. Hagg.<ref>Bishop 1995,{{page needed|date= January 2012}}</ref>}} For more versatility, Bishop made provision for four 20&nbsp;mm cannon in the forward half of the [[bomb bay]], under the cockpit, firing via blast tubes and troughs under the fuselage.<ref name="Bowman 2005, p. 11.">Bowman 2005, p. 11.</ref>

The DH.98 was too radical for the ministry, which wanted a heavily armed, multirole aircraft, combining medium bomber, reconnaissance, and general-purpose roles, that was also capable of carrying torpedoes.{{Sfn | Batchelor | Lowe | 2008 | p = 6}}  With the outbreak of war, the ministry became more receptive, but was still sceptical about an unarmed bomber. They thought the Germans would produce fighters that were faster than had been expected.<ref>Sharp and Bowyer 1971, pp. 31–32,</ref> and suggested the incorporation of two forward- and two rear-firing machine guns for defence.<ref name="Bow0510">Bowman 2005, p. 10.</ref> The ministry also opposed a two-man bomber, wanting at least a third crewman to reduce the work of the others on long flights.<ref name="Bowman 2005, p. 11."/> The Air Council added further requirements such as remotely controlled guns, a top speed of {{cvt|275|mph|km/h|abbr=on|round=5}} at 15,000&nbsp;ft on two-thirds engine power, and a range of {{cvt|3000|mi|km|abbr=on}} with a 4,000-lb bomb load.<ref name="Bow0510"/> To appease the ministry, de Havilland built mock-ups with a gun turret just aft of the cockpit, but apart from this compromise, de Havilland made no changes.<ref name="Bowman 2005, p. 11."/>

On 12 November, at a meeting considering fast-bomber ideas put forward by de Havilland, [[Blackburn Aircraft|Blackburn]], and [[Bristol Aeroplane Company|Bristol]], Air Marshal Freeman directed de Havilland to produce a fast aircraft, powered initially by Merlin engines, with options of using progressively more powerful engines, including the [[Rolls-Royce Griffon]] and the [[Napier Sabre]].  Although estimates were presented for a slightly larger Griffon-powered aircraft, armed with a four-gun tail turret, Freeman got the requirement for defensive weapons dropped, and a draft requirement was raised calling for a high-speed, light-reconnaissance bomber capable of {{cvt|400|mph|km/h|abbr=on|round=5}} at 18,000&nbsp;ft.<ref>Buttler 2004, p. 79.</ref>

On 12 December, the [[Vice-Chief of the Air Staff (United Kingdom)|Vice-Chief of the Air Staff]], Director General of Research and Development, and the [[Air Officer Commanding |Air Officer Commanding-in-Chief]] (AOC-in-C) of [[RAF Bomber Command]] met to finalise the design and decide how to fit it into the RAF's aims.  The AOC-in-C would not accept an unarmed bomber, but insisted on its suitability for reconnaissance missions with F8 or [[F24 camera]]s.<ref name="Bowman 2005, p. 12.">Bowman 2005, p. 12.</ref> After company representatives, the ministry, and the RAF's operational commands examined a full-scale mock-up at Hatfield on 29 December 1939, the project received backing.<ref name= "Bu80">Buttler 2004, p. 80.</ref> This was confirmed on 1 January 1940, when Freeman chaired a meeting with Geoffrey de Havilland, John Buchanan (Deputy of Aircraft Production), and John Connolly (Buchanan's chief of staff). De Havilland claimed the DH.98 was the "fastest bomber in the world… it must be useful".  Freeman supported it for RAF service, ordering a single prototype for an unarmed bomber to specification B.1/40/dh, which called for a light bomber/reconnaissance aircraft powered by two {{cvt |1280|hp|kW|abbr= on}} Rolls-Royce RM3SM (an early designation for the Merlin 21) with [[Meredith effect|ducted radiators]], capable of carrying a {{cvt|1000|lb|kg|adj=on}} bomb load.<ref name="Sharb34"/><ref name="Bowman 2005, p. 12." /> The aircraft was to have a speed of {{cvt|400|mph|km/h}} at {{cvt|24000|ft|m}} and a cruising speed of {{cvt|325|mph|km/h|round=5}} at {{cvt|26500|ft|m}} with a range of {{cvt|1500|mi|km}} at {{cvt|25000|ft|m}} on full tanks. Maximum service ceiling was to be {{cvt|32000|ft|m}}.<ref name= "Bowman 2005, p. 12." />

On 1 March 1940, [[Air Marshal]] [[Roderic Hill]] issued a contract under Specification B.1/40, for 50 bomber-reconnaissance variants of the DH.98; this contract included the prototype, which was given the [[United Kingdom aircraft test serials|factory serial]] ''E-0234''.<ref>Sharp and Bowyer 1971, p. 35.</ref><ref>Price 2009, p. 51.</ref> In May 1940, specification F.21/40 was issued, calling for a long-range fighter armed with four 20&nbsp;mm cannon and four .303 machine guns in the nose, after which de Havilland was authorised to build a prototype of a fighter version of the DH.98. After debate, it was decided that this prototype, given the [[United Kingdom military aircraft serials|military serial number]] ''W4052'', was to carry [[aircraft interception radar|aircraft interception (AI)]] Mk&nbsp;IV radar equipment as both a day fighter and night fighter.{{Efn |The term [[radar]], which originated in the USA, was not used by the British until later.}} By June 1940, the DH.98 had been named "Mosquito".<ref name= "Bu80" /> Having the fighter variant kept the Mosquito project alive, as doubts remained within the government and Air Ministry regarding the usefulness of an unarmed bomber, even after the prototype had shown its capabilities.<ref name="Bu80"/>

===Project Mosquito===
[[File:DE HAVILLAND 1943 Advertisement s.jpg|thumb|upright|1943 advertisement for de Havilland taken from ''[[Flight International|Flight & Aircraft Engineer]]'' magazine highlights the speed of the B Mk.IV.]]
With design of the DH.98 started, mock-ups were built, the most detailed at Salisbury Hall, where ''E-0234'' was later constructed. Initially, the concept was for the crew to be enclosed in the fuselage behind a [[stepless cockpit|transparent nose]] (similar to the [[Bristol Blenheim]] or [[Heinkel He 111]]H), but this was quickly altered to a more solid nose with a conventional canopy.<ref>Thirsk 2006, p. 18.</ref>

Work was cancelled again after the [[Dunkirk evacuation|evacuation of the British Army from France]], when [[Max Aitken, 1st Baron Beaverbrook|Lord Beaverbrook]], as [[Minister of Aircraft Production]], concentrating production on aircraft types for the defence of the UK decided no production capacity remained for aircraft like the DH.98, which was not expected to be in service until early 1942. Beaverbrook told Air Vice-Marshal Freeman that work on the project should stop, but he did not issue a specific instruction, and Freeman ignored the request.<ref>Sharp and Bowyer 1995, pp. 35, 38.</ref> In June 1940, however, Lord Beaverbrook and the Air Staff ordered that production should concentrate on five existing types, namely the [[Supermarine Spitfire]], [[Hawker Hurricane]] fighter, [[Vickers Wellington]], [[Armstrong-Whitworth Whitley]], and [[Bristol Blenheim]] bombers.<ref name="Bowman 2005, p. 12." /> Work on the DH.98 prototype stopped. Apparently, the project shut down when the design team were denied materials for the prototype.<ref name="Jackson 2003, p. 7.">Jackson 2003, p. 7.</ref>

The Mosquito was only reinstated as a priority in July 1940, after de Havilland's general manager, L.C.L. Murray, promised Lord Beaverbrook 50 Mosquitoes by December 1941. This was only after Beaverbrook was satisfied that Mosquito production would not hinder de Havilland's primary work of producing [[Tiger Moth]] and [[Airspeed Oxford]] trainers, repairing [[Hawker Hurricanes|Hurricanes]], and manufacturing Merlin engines under licence.<ref name="Jackson 2003, p. 7."/> In promising Beaverbrook such a number by the end of 1941, de Havilland was taking a gamble, because they were unlikely to be built in such a limited time. As it transpired, only 20 aircraft were built in 1941, but the other 30 were delivered by mid-March 1942.<ref>Sharp and Bowyer 1971, pp. 35–36, 82.</ref> During the [[Battle of Britain]], interruptions to production due to air raid warnings caused nearly a third of de Havilland's factory time to be lost.<ref>Sharp and Bowyer 1971, p. 38.</ref> Nevertheless, work on the prototype went ahead quickly at Salisbury Hall since ''E-0234'' was completed by November 1940.<ref>Thirsk 2006, p. 19.</ref>

In the aftermath of the Battle of Britain, the original order was changed to 20 bomber variants and 30 fighters. Whether the fighter version should have dual or single controls, or should carry a turret, was still uncertain, so three prototypes were built: ''W4052'', ''W4053'', and ''W4073''. The second and third, both turret armed, were later disarmed, to become the prototypes for the T.III trainer.<ref>Thirsk 2006, p. 178.</ref> This caused some delays, since half-built wing components had to be strengthened for the required higher combat loading.  The nose sections also had to be changed from a design with a clear [[Poly(methyl methacrylate)|perspex]] bomb-aimer's position, to one with a solid nose housing four .303 machine guns and their ammunition.<ref name="Batchelor and Lowe 2008, p. 6."/>

===Prototypes and test flights===
On 3 November 1940, the prototype aircraft, painted in "prototype yellow" and still coded ''E-0234'', was dismantled, transported by road to [[Hatfield, Hertfordshire|Hatfield]] and placed in a small, blast-proof assembly building.  Two Merlin 21 two-speed, single-stage supercharged engines were installed, driving three-bladed de Havilland Hydromatic constant-speed [[Variable-pitch propeller (aeronautics)|controllable-pitch propellers]]. Engine runs were made on 19 November.<ref name="Bowman 2005, p. 18."/> On 24 November, taxiing trials were carried out by [[Geoffrey de Havilland Jr.]], the de Havilland [[test pilot]]. On 25 November, the aircraft made its first flight, piloted by de Havilland Jr., accompanied by John E. Walker, the chief engine installation designer.{{Efn|This first flight was made only 11 months after the start of detailed design work.<ref name= "Batchelor and Lowe 2008, p. 6." />}}

For this maiden flight, ''E-0234'', weighing {{cvt|14150|lb|kg}}, took off from the grass airstrip at Hatfield. The takeoff was reported as "straightforward and easy" and the undercarriage was not retracted until a considerable altitude was attained.<ref name="Thir25">Thirsk 2006, p. 25.</ref> The aircraft reached {{cvt|220|mph|abbr=on|round=5}}, with the only problem being the undercarriage doors, which were operated by [[bungee cord]]s attached to the main undercarriage legs, and remained open by some {{cvt |12|in|mm}} at that speed.<ref name="Thir25"/> This problem persisted for some time. The left wing of ''E-0234'' also had a tendency to drag slightly to port, so a slight change in the angle of the wing was carried out before further flights.<ref name="Bowman 2005, p. 18.">Bowman 2005, p. 18.</ref>{{Efn|''E-0234'' had a wingspan of {{cvt|52|ft|2|in|abbr=on}} and was the only Mosquito to be built with Handley Page [[Leading edge slats|slats]] on the outer leading edges of the wings. Test flights showed that these were not needed and they were disconnected and faired over with doped fabric (these slats can still be seen on ''W4050'').<ref>Thirsk 2006, pp. 33, 269.</ref>}}

[[File:De Havilland Mosquito Prototype.jpg|thumb|left|Mosquito prototype ''W4050'' landing after a test flight on 10 January 1941: Four test flights were flown that day.<ref name="Thirsk 2006, p. 31">Thirsk 2006, p. 31.</ref>]]
On 5 December 1940, the prototype, with the [[United Kingdom military aircraft serials|military serial number]] ''W4050'', experienced tail buffeting at speeds between {{cvt|240|and|255|mph|round=5}}. The pilot noticed this most in the control column, with handling becoming more difficult. During testing on 10 December, wool tufts were attached to suspect areas to investigate the direction of airflow. The conclusion was that the airflow separating from the rear section of the inner engine [[nacelle]]s was disturbed, leading to a localised [[Stall (flight)|stall]] and the disturbed airflow was striking the tailplane, causing buffeting. To smooth the air flow and deflect it from forcefully striking the tailplane, nonretractable [[Leading edge slot|slots]] fitted to the inner engine nacelles and to the leading edge of the tailplane were tested.<ref>Thirsk 2006, pp. 27–29.</ref> These slots and wing-root fairings fitted to the forward fuselage and leading edge of the radiator intakes stopped some of the vibration experienced, but did not cure the tailplane buffeting.<ref>Thirsk 2006, pp. 28–29.</ref>

In February 1941, buffeting was eliminated by incorporating triangular fillets on the trailing edge of the wings and lengthening the nacelles, the trailing edge of which curved up to fair into the fillet some {{cvt|10|in|abbr=on}} behind the wing's trailing edge; this meant the [[Flap (aircraft)|flaps]] had to be divided into inboard and outboard sections.<ref name="Bowman 2005, p. 19.">Bowman 2005, p. 19.</ref>{{Efn|On ''W4050'', the nacelle extensions were not as long as those fitted to production Mosquitoes; nacelles of the definitive shape and length were first fitted to the fighter prototype ''W4052''.  The extended nacelles and fillets were not fitted to the first nine PR Mk&nbsp;Is, some of which were converted to B Mk&nbsp;IV series 1 bombers.<ref name="Thirsk 2006, p. 31"/>}} With the buffeting problems largely resolved, [[John Cunningham (RAF officer)|John Cunningham]] flew ''W4050'' on 9 February 1941. He was greatly impressed by the "lightness of the controls and generally pleasant handling characteristics". Cunningham concluded that when the type was fitted with [[Airborne Interception radar|AI]] equipment, it might replace the [[Bristol Beaufighter]] night fighter.<ref name="Bowman 2005, p. 19."/>

During its trials on 16 January 1941, ''W4050'' outpaced a Spitfire at {{cvt|6000|ft|abbr=on}}. The original estimates were that as the Mosquito prototype had twice the surface area and over twice the weight of the [[Supermarine Spitfire (early Merlin powered variants)|Spitfire Mk.II]], but also had twice its power, the Mosquito would be {{cvt|20|mph|km/h|round=5}} faster. Over the next few months, ''W4050'' surpassed this estimate, easily beating the Spitfire Mk.II in tests at [[RAF Boscombe Down]] in February 1941, reaching a top speed of {{cvt|392|mph|km/h}} at {{cvt |22000|ft|m}}, compared to a top speed of {{cvt|360|mph|km/h}} at {{cvt|19500|ft|m}} for the Spitfire.<ref name="Bowman 2005, p. 8."/>

On 19 February, official trials began at the [[Aeroplane and Armament Experimental Establishment]] (AAEE) based at Boscombe Down, although the de Havilland representative was surprised by a delay in starting the tests.<ref name="Sharb445">Sharp and Bowyer 1971, pp. 44–45.</ref> On 24 February, as ''W4050'' taxied across the rough airfield, the tailwheel jammed leading to the fuselage fracturing. Repairs were made by early March, using part of the fuselage of the photo-reconnaissance prototype ''W4051''.{{Efn|This accident occurred because the tailwheel unit was unable to [[caster]] properly, a problem solved in June–July 1941 by fitting a [[Dowty Group|Dowty]] unit.}} In spite of this setback, the ''Initial Handling Report 767'' issued by the AAEE stated, "The aeroplane is pleasant to fly… aileron control light and effective{{nbsp}}..." The maximum speed reached was {{cvt |388|mph}} at {{cvt|22000|ft}}, with an estimated maximum ceiling of {{cvt|34000|ft}} and a maximum rate of climb of {{cvt|2880|ft/min}} at {{cvt |11500|ft}}.<ref name="Sharb445" />

''W4050'' continued to be used for various test programmes, as the experimental "workhorse" for the Mosquito family.<ref>Thirsk 2006, pp. 15–16.</ref> In late October 1941, it returned to the factory to be fitted with [[Rolls-Royce Merlin|Merlin 61s]], the first production Merlins fitted with a two-speed, two-stage supercharger. The first flight with the new engines was on 20 June 1942.<ref>Sharp and Bowyer 1971, p. 47.</ref> ''W4050'' recorded a maximum speed of {{cvt|428|mph|abbr=on}} at {{cvt|28500|ft|abbr=on}} (fitted with straight-through air intakes with snow guards, engines in full supercharger gear) and {{cvt|437|mph|abbr=on}} at {{cvt|29200|ft|abbr=on}} without snow guards.<ref name="Thr34">Thirsk 2006, p. 34.</ref>{{Efn |Snowguards were small, oval grids of steel mesh attached in front of the opening of the carburettor air intakes. They were designed to stop ice forming in the mouth of the intake when the aircraft operated in cold weather, or flew in the cold temperatures at high altitude.}} In October 1942, in connection with development work on the NF Mk.XV, ''W4050'' was fitted with extended wingtips, increasing the span to {{cvt |59|ft|2|in|abbr=on}}, first flying in this configuration on 8 December.<ref>Thirsk 2006, p. 35.</ref> Fitted with high-altitude-rated, two-stage, two-speed Merlin&nbsp;77s, it reached {{cvt|439|mph|km/h}} in December 1943.<ref name="Bowman 2005, p. 21." /> Soon after these flights, ''W4050'' was grounded and scheduled to be scrapped, but instead served as an instructional airframe at Hatfield. In September 1958, ''W4050'' was returned to the Salisbury Hall hangar where it was built, restored to its original configuration, and became one of the primary exhibits of the [[de Havilland Aircraft Heritage Centre]].<ref>Thirsk 2006, 36–38.</ref><ref>[https://www.youtube.com/watch?v=gfXNXGGNRws Time-lapse footage of restoration work on ''W4050'', 9 January 2011.] {{Webarchive|url=https://web.archive.org/web/20170305121925/https://www.youtube.com/watch?v=gfXNXGGNRws&feature=related |date=5 March 2017 }} Retrieved: 3 January 2012.</ref>

[[File:De Havilland Mosquito - Prototype 1.jpg|thumb|''W4050'' being restored at the [[de Havilland Aircraft Heritage Centre]] near St Albans]]
''W4051'', which was designed from the outset to be the prototype for the photo-reconnaissance versions of the Mosquito, was slated to make its first flight in early 1941. However, the fuselage fracture in ''W4050'' meant that ''W4051's'' fuselage was used as a replacement; ''W4051'' was then rebuilt using a production standard fuselage and first flew on 10 June 1941. This prototype continued to use the short engine nacelles, single-piece trailing-edge flaps, and the {{cvt|19|ft|5.5|in|abbr=on}} "No. 1"  tailplane used by ''W4050'', but had production-standard {{cvt|54|ft|2|in|abbr=on}} wings and became the only Mosquito prototype to fly operationally.<ref name="Thirsk 2006, p. 269">Thirsk 2006, p. 269.</ref>

Construction of the fighter prototype, ''W4052'', was also carried out at Salisbury Hall. It was powered by {{cvt|1460|hp|kW|abbr=on}} Merlin&nbsp;21s, and had an altered canopy structure with a flat, bullet-proof windscreen; the solid nose had mounted four .303 British [[M1919 Browning machine gun|Browning]] machine guns and their ammunition boxes, accessible by a large, sideways hinged panel.<ref>Thirsk 2006, p. 125.</ref> Four 20-mm [[Hispano-Suiza HS.404|Hispano&nbsp;Mk.II]] cannon were housed in a compartment under the cockpit floor with the breeches projecting into the bomb bay and the automatic bomb bay doors were replaced by manually operated bay doors, which incorporated cartridge ejector chutes.<ref name="Thirsk 2006, pp. 125, 134">Thirsk 2006, pp. 125, 134.</ref>

As a day and night fighter, prototype ''W4052'' was equipped with AI Mk&nbsp;IV equipment, complete with an "arrowhead" transmission aerial mounted between the central Brownings and receiving aerials through the outer wing tips, and it was painted in black RDM2a "Special Night" finish.<ref>Thirsk 2006, pp. 124–27.</ref>{{Efn|The RDM2a finish was notoriously rough and hard to maintain; tests later proved that it could cut the top speed of a Mosquito by up to {{cvt|26|mph|abbr=on}}, and by March 1942 it was replaced by the black "Night Type S" (smooth) finish.<ref>Thirsk 2006, p. 134.</ref>}} It was also the first prototype constructed with the extended engine nacelles.<ref>Thirsk 2006, p. 126.</ref> ''W4052'' was later tested with other modifications, including bomb racks, drop tanks, [[barrage balloon]] cable cutters in the leading edge of the wings, Hamilton airscrews and braking propellers, and drooping aileron systems that enabled steep approaches and a larger rudder tab. It continued to serve as a test machine until it was scrapped on 28 January 1946.<ref name="Bowman 2005, p. 21." /> ''4055'' flew the first operational Mosquito flight on 17 September 1941.<ref name="Bowman 2005, p. 20." />

During flight testing, the Mosquito prototypes were modified to test a number of configurations. ''W4050'' was fitted with a turret behind the cockpit for drag tests, after which the idea was abandoned in July 1941. ''W4052'' had the first version of the [[Fairey Aviation Company|Youngman Frill]] airbrake fitted to the fighter prototype. The frill was mounted around the fuselage behind the wing and was opened by bellows and [[venturi effect]] to provide rapid deceleration during interceptions and was tested between January and August 1942, but was also abandoned when lowering the undercarriage was found to have the same effect with less buffeting.<ref name="Bowman 2005, p. 20."/>
<!--- Something is missing here--- It was used to eliminate problems on ''W4052''. Most of this related to the former's exhaust and armament systems. Flash eliminators had to be fitted to the NF.II to prevent the crew from being blinded by muzzle flash during the firing of the weapons.--->

===Production plans and American interest===
The Air Ministry authorised mass production plans on 21 June 1941, by which time the Mosquito had become one of the world's fastest operational aircraft.{{Sfn | Bowman | 2005 | p = 21}}  It ordered 19 photo-reconnaissance (PR) models and 176 fighters. A further 50 were unspecified; in July 1941,  these were confirmed to be unarmed fast bombers.{{Sfn | Bowman | 2005 | p = 21}}  By the end of January 1942, contracts had been awarded for 1,378 Mosquitoes of all variants, including 20 T.III trainers and 334 FB.VI bombers. Another 400 were to be built by [[de Havilland Canada]].<ref name= "Bowman 2005, p. 24.">{{Harvnb | Bowman | 2005 | p = 24}}.</ref>

On 20 April 1941, ''W4050'' was demonstrated to Lord Beaverbrook, the [[Minister of Aircraft Production]]. The Mosquito made a series of flights, including one rolling climb on one engine. Also present were US General [[Henry H. Arnold]] and his aide [[Major (rank)|Major]] [[Elwood Quesada]], who wrote "I… recall the first time I saw the Mosquito as being impressed by its performance, which we were aware of. We were impressed by the appearance of the airplane that looks fast usually is fast, and the Mosquito was, by the standards of the time, an extremely well-streamlined airplane, and it was highly regarded, highly respected."{{Sfn | Bowman | 2005 | p = 19}}<ref>[https://www.youtube.com/watch?v=d7pNBSRkEiQ "WWII US Film on Mosquito."] {{Webarchive |url=https://web.archive.org/web/20170219201139/https://www.youtube.com/watch?v=d7pNBSRkEiQ&feature=related |date= 19 February 2017 }} ''You tube.'' Retrieved: 3 January 2012.</ref>

The trials set up future production plans between Britain, [[Australia]], and [[Canada]]. Six days later, Arnold returned to America with a full set of manufacturer's drawings. As a result of his report, five companies (Beech, [[Curtiss-Wright]], [[Fairchild Aircraft|Fairchild]], Fleetwings, and Hughes) were asked to evaluate the de Havilland data. The report by Beech Aircraft summed up the general view: "It appears as though this airplane has sacrificed serviceability, structural strength, ease of construction and flying characteristics in an attempt to use construction material which is not suitable for the manufacture of efficient airplanes."<ref>{{Cite book|title=Mosquito the Original Multi Role Aircraft|last=Simons|first= Graham M. |publisher=Pen & Sword Aviation|year=2011|isbn=978-1-84468260-7}}</ref>{{page needed|date=October 2022}} The Americans did not pursue the proposal for licensed production, the consensus arguing that the [[Lockheed P-38 Lightning]] could fulfill the same duties.  However, Arnold urged the [[United States Army Air Forces]] (USAAF) to evaluate the design even if they would not adopt it. On 12 December 1941, after the [[attack on Pearl Harbor]], the USAAF requested one airframe for this purpose.<ref name="Bowman 2005, p. 20.">Bowman 2005, p. 20.</ref>