Editing Short Stirling (section)

===Origins===
In the 1930s, the Royal Air Force (RAF) was interested primarily in twin-engined [[bombers]].<ref name = "norris 3"/> These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. Power limitations were so serious that the British invested in the development of huge engines in the 2,000-[[horsepower]] (1,500&nbsp;kW) class to improve performance. During the late 1930s, none of these were ready for production. The [[United States]] and the [[Soviet Union]] were pursuing the development of bombers powered by arrangements of four smaller engines; the results of these projects proved to possess favourable characteristics such as excellent range and fair lifting capacity, and in 1936, the RAF also decided to investigate the feasibility of the four-engined bomber.<ref name = "norris 3"/>

The Air Ministry published [[List Of Air Ministry Specifications|Specification B.12/36]], for a high-speed, long-range, four-engined [[strategic bomber]] aircraft that would be capable of being designed and constructed at speed.<ref name = "norris 3"/> The bomb load was to be a maximum of {{convert|14,000|lb|kg}} carried to a range of {{convert|2,000|mi|km}} or a lesser payload of {{convert|8,000|lb|kg}} to {{convert|3,000|mi|km}} (very demanding for the era). It was to have a crew of six and was to have a normal all-up weight of 48,000&nbsp;lb, while a maximum overload weight of 65,000&nbsp;lb was also envisioned.<ref name = "norris 3"/> The aircraft would have to be capable of cruising at speeds of 230&nbsp;mph or greater while flying at {{convert|15000|ft|m|abbr=on}}, carrying three gun turrets (located in the nose, amidships, and rear positions) for defence.<ref name="ButtlerSecret">{{Harvnb|Buttler|2004|p=96}}</ref>

The aircraft should also be able to be used as a troop transport for 24 soldiers and be able to use catapult assistance for takeoff when heavily laden.<ref name="Barnes p371">{{Harvnb|Barnes|1967|p=371.}}</ref><ref name="ButtlerSecret"/> The concept was that the aircraft would fly troops to far corners of the [[British Empire]] and then support them with bombing. To help with this task, as well as ease production, it needed to be able to be broken down into parts, for transport by train.<ref name= "Flight p. 96">{{Harvnb|''Flight'' 29 January 1942|p=96}}</ref> Since it could be operating from limited "back country" airfields, it needed to lift off from a {{convert|500|ft|m|abbr=on}} runway and be able to clear {{convert|50|ft|m|abbr=on}} trees at the end, a specification with which most small aircraft would have a problem today. Aviation author Geoffrey Norris observed that the stringent requirements given in the specification for the prospective aircraft to be able to make use of existing infrastructure, specifically the specified maximum wingspan of 100 feet, adversely affected the Stirling's performance, such as its relatively low ceiling and its inability to carry anything larger than 500&nbsp;lb bombs.<ref name = "norris 3">{{harvnb|Norris|1966|p=3}}</ref>

In mid-1936, Specification B.12/36 was sent out to [[Supermarine]], [[Boulton Paul]], [[Handley Page]], and [[Armstrong Whitworth]]. In August, the specification was issued to the rest of British industry.<ref name = "ButtlerSecret"/> Left out of those asked to tender designs, Shorts were later included because the company already had similar designs in hand while possessing ample design staff and production facilities to fulfil production commitments. Shorts were producing several four-engined [[flying boat]] designs of the required size and created their S.29 proposal by removing the lower deck and boat hull of the [[Short Sunderland|S.25 Sunderland]]. The new S.29 design was similar to the Sunderland; the wings and controls were the same, construction was identical, and it even retained the slight upward bend at the rear of the fuselage, which had originally been intended to keep the Sunderland's tail clear of sea spray. As originally designed, the S.29 was considered to be capable of favourable high-altitude performance.<ref name = "norris 3"/>

Following a Tender Design Conference in October 1936, the S.29 was low down on the short list of designs considered. Vickers Type 293 submission was first followed by the Boulton Paul P.90, Armstrong Whitworth's AW.42, the Supermarine Type 316, and then the Short S.29. The Supermarine was ordered in prototype (two aircraft) form as the revised [[Supermarine Type 317]]{{refn|group=N|The Type 317 had increased wing area over the Type 316 and twin fins. Supermarine were told not to proceed further with their Merlin-engined alternative, the Type 318, to concentrate on the Type 317}} in January 1937. An alternative design to the Supermarine was needed for insurance, and Shorts should build it, as they had experience with four-engined aircraft. The original design had been criticised when considered, and in February 1937, the Air Ministry suggested modifications to the design, including considering the use of the [[Bristol Hercules]] radial engine as an alternative to the [[Napier Dagger]] inline, increasing service ceiling to 28,000&nbsp;ft (carrying a 2000&nbsp;lb of bombs) and reducing the wingspan.<ref>{{Harvnb|Buttler|2004|p=98}}</ref> Shorts accepted this large amount of redesign work. The project had added importance due to the death of Supermarine's designer, [[Reginald Mitchell]], which had generated doubt within the Air Ministry about the ability of Supermarine to deliver.<ref>{{Harvnb|Buttler|2004|p=99}}</ref> Two prototypes were ordered from Shorts.{{sfn|Buttler|2004|p=99}}

The S.29 used the Sunderland's {{convert|114|ft|m|abbr=on}} wing and it had to be reduced to less than {{convert|100|ft|m|abbr=on}}{{refn|The contemporary P.13/36 specification that led to the Handley Page Halifax and Avro Manchester was for twin-engined aircraft that were expected to be same weight as the [[Vickers Warwick|B.1/35 bomber specification]], which was also limited to 100 ft wingspan, but "a good deal smaller and faster".{{sfn|Buttler|2004|p=101}}|group=N}}<ref>{{Harvnb|Mondey|1994|p=189.}}</ref><ref>{{harvnb|Winchester|2005|p=48.}}</ref><ref name = "norris 3"/> To get the needed lift from a shorter span and excess weight, the redesigned wing was thickened and reshaped.<ref name = "norris 3"/> The wingspan was often said to be limited to 100&nbsp;ft so the aircraft would fit into existing hangars, but the maximum hangar opening was {{convert|112|ft|m|abbr=on}} and the specification required outdoor servicing.<ref name= "Flight p. 96"/> The wingspan limit was a method of stopping aircraft from being too large.<ref>{{Harvnb|Buttler|2004|p=100}}</ref> In June 1937, the S.29 was accepted as the second string for the Supermarine Type 317 and formally ordered in October; Shorts and Supermarine were issued with instructions to proceed.<ref name = "norris 3"/>