Editing Vickers Wellington (section)

===Origins===
[[File:Wellingtons under construction WWII IWM CH 5980.jpg|thumb|Wellingtons under construction, showing the [[geodetic airframe]]]]
[[File:New Zealand Wellington Bombers in England1939.JPG|thumb|[[RNZAF]] Wellington Mark I aircraft with the original turrets; anticipating war, the [[New Zealand]] government loaned these aircraft and their aircrews to the RAF in August 1939]]
In October 1932, the British [[Air Ministry]] invited Vickers to tender for the recently issued [[List of Air Ministry specifications|Specification B.9/32]], which sought a twin-engine medium daylight bomber. In response, Vickers conducted a design study, led by Chief Designer [[Rex Pierson]].<ref>{{cite magazine|title=Remembering Rex Pierson|magazine=Flight (and Aircraft Engineer)|date=21 November 1952|volume=LXII|issue=2287|page=651|url=https://www.flightglobal.com/pdfarchive/view/1952/1952%20-%203355.html|access-date=6 November 2015
|archive-url= https://web.archive.org/web/20160304121006/https://www.flightglobal.com/pdfarchive/view/1952/1952%20-%203355.html |archive-date= 2016-03-04}}</ref> Early on, Vickers' chief structures designer [[Barnes Wallis]] proposed the use of a [[geodetic airframe]], inspired by his previous work on [[airship]]s and the single-engined [[Vickers Wellesley|Wellesley]] [[light bomber]].<ref name = "andrew 3">Andrews 1967, p. 3.</ref> During structural testing performed at the [[Royal Aircraft Establishment]], [[Farnborough Airfield|Farnborough]], the proposed structure demonstrated not only the required strength factor of six, but reached 11 without any sign of failure, proving the geodetic airframe to possess a strength far in excess of normal levels.<ref name = "andrew 3"/> This strength allowed for the structure design to be further developed to reduce the size of individual members and adopt simplified standard sections of lighter construction.<ref name = "andrew 3"/>

Vickers studied and compared the performance of various [[air cooling|air-]] and [[Radiator (engine cooling)|liquid-cooled]] engines to power the bomber, including the [[Bristol Pegasus|Bristol Pegasus IS2]], Pegasus IIS2, and [[Armstrong Siddeley Tiger]] radials, and the [[Rolls-Royce Goshawk|Rolls-Royce Goshawk I]] inline.<ref name = "andrew 5"/> The Pegasus was selected as the engine for air-cooled versions of the bomber, while the Goshawk engine was chosen for the liquid-cooled engine variant. On 28 February 1933, two versions of the aircraft, one with each of the selected powerplants, were submitted to the tender.<ref name = "andrew 5"/> In September 1933, the Air Ministry issued a pilot contract for the Goshawk-powered version.<ref name = "andrew 5"/> In August 1934, Vickers proposed to use either the Pegasus or the [[Sleeve valve|sleeve-valve]] 9-cylinder radial [[Bristol Perseus]] engines instead of the evaporative-cooled Goshawk, which promised improvements in speed, climb rate, ceiling, and single-engine flight capabilities without any major increase in all-up weight; the Air Ministry accepted the proposed changes.<ref name = "andrew 6">Andrews 1967, p. 6.</ref>

Other refinements of the design had also been implemented and approved, such as the adoption of [[Variable-pitch propeller (aeronautics)|variable-pitch propeller]]s, and the use of Vickers-produced [[Gun turret#Aircraft|gun turrets]] in the nose and tail positions.<ref name = "andrew 6"/> By December 1936, the specification had been revised to include front, rear, and midship wind-protected turret mountings.<ref name = "andrew 5"/> Other specification changes included modified bomb undershields and the inclusion of spring-loaded bomb bay doors.<ref name = "andrew 5 6">Andrews 1967, pp. 5–6.</ref> The proposal had also been developed further, a mid-wing arrangement was adopted instead of a shoulder-mounted wing for greater pilot visibility during [[formation flight]] and improved aerodynamic performance, as well as a substantially increased overall weight of the aircraft.<ref name = "andrew 5 6"/> Design studies were also conducted on behalf of the Air Ministry into the adoption of the [[Rolls-Royce Merlin]] engine.<ref name = "andrew 6"/>

In spite of a traditional preference of the establishment to strictly adhere to the restrictive [[tare weight]] for the aircraft established in the tender, both Pierson and Wallis firmly believed that their design should adopt the most powerful engine available.<ref name = "andrew 3"/> Perhaps in response to pressure from Vickers, the Air Ministry overlooked, if not openly accepted, the removal of the tare weight restriction, as between the submission of the tender in 1933 and the flight of the first prototype in 1936, the tare weight eventually rose from {{convert|6,300|lb|t|abbr=on}} to {{convert|11,508|lb|t|abbr=on}}.<ref name = "andrew 3 5 6">Andrews 1967, pp. 3, 5–6.</ref>{{efn |Tony Buttler notes that Specification B.3/34 had a higher tare weight because of the lapse of international agreement in 1934 on the weights on bombers and Vickers declined to tender to the specification because they were already scaling up the Type 271 design to where it would meet the specification<ref>Buttler, 2004. p. 93</ref> }} The prescribed bomb load and range requirements were routinely revised upwards by the Air Ministry; by November 1935, figures within the Ministry were interested in the possibility of operating the aircraft at an all-up weight of {{convert|30,500|lb|t|abbr=on}}, which aviation author C.F. Andrews described as "a very high figure for a medium bomber of those days".<ref name = "andrew 3"/>

During the development phase of the aircraft, as C F Andrews puts it "the political and military climate of Europe was changing rapidly. The threats of the dictators of Germany and Italy began to exert pressure on the British Government to make a reappraisal of the strength of its armed forces, especially that of the Royal Air Force".<ref name = "andrew 3"/> By 1936, the need for a high priority to be placed on the creation of a large bomber force, which would form the spearhead of British offensive power, had been recognised; accordingly, a new command organisation within the RAF, [[RAF Bomber Command|Bomber Command]], was formed that year to deliver upon this requirement.<ref name = "andrew 3"/>