The question of the 100' limit dates back to at least 1933 and the Fairey Hendon. A Wing Commander Williams, the Flying Operations 1 (FO1), reported that he saw difficulties with the Hendon's 101' 9" wingspan as it would not fit the RAF's 100' hangars. He suggested a redesign of the Hendon to a 97' wingspan. Air Commodore Mills, the Director of Organisation and Staff Duties (DOSD) pointed out that type 'A'hangars actually had 120' door openings, but that the small clearance which would be achieved at each wing tip (with a 97' span) for the old 100' hangars would be "insufficient for taking large aircraft through a doorway." His comment underlines the absurdity of claims that a 100' span limit was introduced with a view to getting large aircraft into hangar with a door opening of exactly the same size. The Deputy Chief of the Air Staff, then Ludlow-Hewitt, commented that he had seen a Hendon put on a "skate" and pushed sideways into a hangar with an 85' doorway, and suggested further investigation of this technique.
The 100' limit was introduced in 1936 simply to stop the aircraft getting too large. It was not an economic initiative. It was the result of concerns arising from the difficulty in handling two large transports built to C.16/28, which were deemed to unwieldy for ground handling.
Financially the Air Ministry worked on a bomb lift per pound (sterling) , for example in the ideal Bomber programme the equation was bomb lift per £20 million.
Once the bombers' targets were obviously going to be in Germany, rather than France which is a lot closer, an increase in size was accepted as inevitable.
The 100' limit was restated in the discussion of the B.1/35 bomber specification.
For the two 1936 bomber specifications (B.12/36 and P.13/36) hangar size was not an issue as the first paragraph of both specifications stated that
"Since it will be required to operate from bases anywhere in the world the aircraft must possess good facilities for maintenance in the open."
B.12/36 further specified that the engines must be easily and quickly replaceable "in the field."
Even more obviously in contradiction to any limit, even on ground handling, in October 1936 the Air Staff proposed that a development of the B.12/36 heavy bomber, with a wingspan not limited to 100' be included in the 1937 Experimental Aircraft Programme.
The myth of the 100' span probably dates to comments made by Barnes, in his book (1960s?) that Short Bros. wanted a 112' wingspan for the Stirling but that
"Unfortunately, before ordering any prototypes the Air Ministry stipulated that the span must not exceed 100', in order to conform to existing hangar dimensions."
This is certainly based on the preview of the requirement, though the final forms sent to Short Bros. included the same. The mention of hangars seems to have been the company's' interpretation, seen by Barnes, as the company sought, later, to justify the poor performance of the Stirling, particularly its low ceiling.
In their response to B.12/36 Short Bros designed a 99' wing with an aspect ratio of 6.71.
Mitchell, for the Supermarine types 316-318 designed first a 93' wing with an aspect ratio 6.97, later increased to 97' with a ratio of 6.93. Mitchell did not feel it necessary to exploit the full span limit set by the Ministry, despite a limitation on span potentially preventing a designer achieving an aspect ration high enough for the required ceiling and range (span squared is the critical parameter).
The major failing of the Stirling was nothing to do with the span limit and everything to do with a huge increase in weight as the design developed, even though the requirement to stress for catapult launch was removed in May 1938. In 1940, when the question of increasing the fuel capacity of the Stirling was being considered, it had grown so heavy that it was found that increasing the fuel capacity
"had no operational advantage since even with no bombs the weight of the aircraft plus normal military equipment exceeded the anticipated practicable operational all up weight."
One final thought. The Lancaster had a wingspan of just 102'.
Cheers
Steve