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The Short Stirling was the first of the famous trio of four engined heavy bombers to enter RAF service. Its performance suffered as a result of restrictions imposed on its design by unrealistic pre-war requirements, and it was never as successful as a heavy bomber as the later Handley Page Halifax or Avro Lancaster.
The Stirling was developed in response to Air Ministry Specification B.12/36 (as was the Boulton Paul P.90, which never reached the prototype stage). This called for a four engined bomber capable of carrying 2,000 lbs of bombs over a range of 1,500 miles with a normal takeoff or 14,000 lbs of bombs for 2,000 miles with a catapult assisted takeoff. The specification also limited the wingspan of the Stirling to 100 feet, later blamed for much of its poor performance (in fact the Stirling’s span of 99 feet 1 in was only three feet smaller than the 102 feet of the Lancaster). Short wanted to use a much wider wing, with a span of 112 feet but permission was refused. The Stirling was designed with small pre-war RAF airfields in mind, and a bomber with a wingspan of 112 feet would have been too large to fit in the hangers and cumbersome around the field. Pre-war conditions also made it desirable that the new heavy bomber would have a short takeoff run.
The basic fuselage design was complete in 1937. It was eight-seven feet long, and made in four segments. One more limit on the future usefulness of the Stirling appeared at this stage. The bomb bay was divided into three parallel sections, each capable of carrying bombs with a diameter of two feet or less. In 1937 this was not seen as a problem – the largest bomb expected to be used was a 2,000 pounder, and most bombs were much smaller. As the war progressed, the size of bombs in use constantly increased, with the standard bomb for most of the war being the 4,000lb “blockbuster”. The Stirling would prove to unable to cope with these larger bombs.
The first prototype for the Stirling was a half scale flying prototype, constructed of plywood and powered by four 90hp engines. The aim was to reveal any problems with the basic design. The half size prototype first flew on 19 September 1938. The only major problem that was detected at this stage was that the takeoff and landing runs threatened to be too long. In order to reduce the takeoff length it was decided to increase the angle between the wing and the ground when landed from 3.5 degrees to 6.5 degrees. This would increase the amount of lift generated early in the take off. This was achieved by increasing the length of the landing gear, giving the Stirling its characteristic high cockpit position – the front of the aircraft was over twenty feet above the ground. This also made it somewhat more difficult to reach the engines, which were located high on the wings.
As work on the Stirling continued, its expected weight kept rising and its performance kept falling. In 1937 it was expected that the Stirling would cruise at 282 mph while carrying 8,000lb of bombs. Tests on the Stirling Mk I later revealed a cruising speed of 165 mph at 10,000 feet, and a maximum speed of only 218 mph, over 100 mph lower than predicted. As well as suffering from its relatively narrow wingspan the Stirling was the heaviest of the three four engined bombers, and at first suffered from its under-powered Hercules II engines.
As a result of tests on the half scale prototype, the Air Ministry decided to place an order for 100 Stirling Mk Is. The first full scale prototype was flow on 14 May 1939, but was destroyed on landing when the light alloy undercarriage gave way. Tests on the early production Stirlings were disappointing, and the early Mk I was relegated to a training role. The Stirling would not fly its first combat mission until February 1941.
