Air Ministry Specification B.12/36 called for the design and development of a strategic four-engined heavy bomber that could quickly is placed into production, giving the RAF Bomber Command a high-speed aircraft capable of delivering a large bombload at long ranges. It was to be crewed by seven or eight men with defensive armament consisting of multi-gunned nose, ventral and tail turrets. The initial maximum take-off weight had to be between 48,000 lbs (21769 kg) and 53,000 lbs (24036 kg), but with the capability of that figure being increased to around 65,000 lbs (29478 kg). The weapons bays also had to be compatible with all standard RAF bomb ordnance in use at that time. The specification also demanded that the aircraft be capable of lifting off a 500 ft (152.4 m) runway and is able to clear 50 ft (15.2 m) trees at the end, with the wingspan not exceeding 100 ft (30.48 m).
Several companies submitted their designs to the Air Ministry, these being Armstrong Whitworth, Short Brothers and Supermarine. The Armstrong Whitworth design was rejected, and both Short Brothers and Supermarine were asked to construct prototypes. Supermarine’s Type 317 prototype was still under construction when the factory was bombed by the Luftwaffe early in the war. The factory and prototype were virtually destroyed, causing Supermarine to withdraw from the competition leaving only the Short Brothers design.
Short initially proposed a design that would give good high-altitude performance provided by a wing spanning 112 ft 0 in (34.14 m) and was to be powered by four Rolls Royce Goshawk engines. Provision was also made for a remote control turret in the lower portion of the rear fuselage. Short would incorporate the same structural and aerodynamic concepts they had used on the Short S.25 Sunderland (maritime reconnaissance flying boat). The RAF rejected this proposal based on the wingspan, demanding it to be made shorter so that the aircraft would fit in RAF aircraft hangers that had standard door openings of 100 ft (30.48 m). This requirement would severely restrict the Stirlings operational altitude. The Short design team had therefore to revise its concept with a wing of reduced span and greater chord, the resulting decrease in aspect ratio inevitably reducing high-altitude capability. Even though this meant a reduction of capabilities of the Stirling, the need for an aircraft of this type was so urgent, the Air Ministry was forced to continue with the project and ordered two prototypes designated Short S.29 Stirling. Production orders for the aircraft followed even before the prototypes flew.
To test the aerodynamics and controllability of the new type, the S.31 was designed as the half-scale prototype with a powerplant of four 90 hp (67 kW) Pobjoy Niagara III radial engines. The Short S.31 made its first flight on 19 September 1938 and revealed good overall handling characteristics. Short had originally decided on an incidence of 3° giving the best possible cruise performance, but the RAF asked that the incidence be increased to 6.5°, being more concerned with improving take-off performance than the cruising speed. In order to accommodate the RAF request for increased wing incidence a major re-design of the central fuselage would have normally be undertaken, but because of time restraints, Short decided on a “quick fix” by lengthening the main landing gear legs to give a higher ground angle.
At the end of 1938, this change was incorporated on the Short S.31 prototype. Other changes included the installation of four 115 hp (86 kW) Pobjoy Niagara IV radial engines. In order to address longitudinal control problems horn-balanced elevators were installed but these were soon replaced by a larger tailplane with conventional elevators.
The construction of two Short S.29 prototypes started in 1939 and the first prototype (L7600) was flown for the first time on 14 May 1939 powered by four 1,375 hp (1025 kW) Bristol Hercules II engines. It would also be its last flight, as on landing one of the wheel brakes seized causing one of the landing gear legs to shear off slamming the aircraft into the ground. Damage was so extensive, the aircraft was written off. The failure was traced to the light alloy undercarriage back arch braces which were replaced on succeeding aircraft by stronger tubular steel units. The main landing gear units on the second prototype (K7605) were redesigned, with this aircraft first flying on 3 December 1939. During the spring of 1940, the prototype spent four months undergoing service tests at Boscombe Down. Main production had already started by this time, with the first Short S.29 Stirling Mk I flying on 7 May 1940 powered by four 1,595 hp (1189 kW) Hercules XI radial engines. The revised landing gear would later give the aircraft a tendency to swing violently unless handled carefully during take-off and landing.
Initial deliveries began in August 1940 to No. 7 Squadron based at Leeming, replacing their Handley Page Hampdens. The Stirling was used operationally for the first time on the night of 10/11 February 1941, when three aircraft from No.7 Squadron attacked oil storage tanks at Rotterdam. The Stirling was thus the RAF’s first four-engined monoplane bomber into service, the first to be used operationally in World War II, and also the first to be withdrawn from the bomber role after a final operational sortie on 8 September 1944. This occurred when there were adequate supplies of the Avro Lancaster and Handley Page Halifax bombers for Bomber Command requirements, for the Stirling had an inadequate operational ceiling and could not carry the larger high-explosive bombs that had been introduced by that time. Total production of bomber versions then amounted to 1,759 aircraft, comprising of the Stirling Mk I (712) and Stirling Mk III (1,047) aircraft. The designation Stirling Mk II was allocated to a planned production version to be built in Canada with 1,600 hp (1193 kW) Wright R-2600-A5B Cyclone radial engines, but this was cancelled after only two prototypes were produced by converting two Mk I aircraft.
As 1942 progressed, No. 7 squadron (with its Stirling’s) and other Bomber Command squadrons were transferred to form the nucleus of the newly formed No. 8 (Pathfinder) Group. By the end of the year, the new Mk.III Stirling’s, equipped with 1,675 hp Bristol Hercules XVI engines and a new dorsal turret design, were entering service and slowly replacing the Mk I’s.
By late 1943, German flak defences were inflicting serious losses to the Stirlings, mostly due to its low ceiling caused by its restricted wing span. It was soon evident that such losses could not continue and Air Marshal Harris, was forced to withdraw the aircraft from operations. By early 1944, as supplies of the Avro Lancaster became available, most of the Stirling squadron’s began to re-equipped with this type. Although, it would not be until September 8, 1944 that No. 149 squadron, flew last operation Stirling sorties against Le Havre.
The Stirling never lived up to it potential as a great bomber (in no small part to the RAF Bomber Commands requirement changes) but it a proved very popular aircraft with its crews, who dubbed it the “fighter bomber” due to its excellent manoeuvrability and rugged construction. On one occasion four German night fighters attacked a Stirling from No. 218 Squadron on a night raid in 1942. Manoeuvring for its life, the Stirling managed to shoot down three of the attackers before returning to base safely, although a little battered. As a result of its high wing loading, the Stirling had a high roll rate and was manoeuvrable enough to out-turn the Junkers Ju 88 and Bf 110 night fighters.
From early 1944 the Stirling’s primary role changed to that of glider tug and transport. For the former role two Stirling Mk IIIs were converted as prototypes, losing their nose and dorsal gun turrets, retaining the tail turret and gaining glider towing equipment to become designated Stirling Mk IV. They proved efficient in this new role, towing one General Aircraft Hamilcar or two Airspeed Horsas for assault and up to five General Aircraft Hotspurs on a ferry flight or for training. The Stirling Mk IV also saw service With No. 100 (Bomber Support) Group, carrying out Electronic Counter Measure (ECM) sorties. They also took part in the D-Day operations in Normandy, in the airborne operations at Arnhem and the March 1945 crossing of the Rhine. Production of the Stirling IV totalled 549.
The Stirling Mk V transport was the last version of the aircraft built for RAF Transport Command. This was configured to carry 40 troops, or 20 fully equipped paratroops, or 12 stretchers and 14 seated casualties. It could be used also for loads such as two jeeps with trailers, or a jeep with a field gun, trailer and ammunition. The Mk Vs were the last Stirlings in service, being gradually replaced by the Avro York, with the last of them withdrawn from use in 1946. Production on Belfast built Mk V totalled 160 aircraft. During 1947 Airtech Limited of Thame, Oxon, converted 12 Stirling Mk Vs for use by a Belgian civil operator under the name Silver Stirling.
Official service figures credit the Stirling with 18,440 sorties flown in which 27,821 tons (28268 tonnes) of bombs were dropped and 20,000 mines were laid, for the loss of 769 aircraft.
Units: They initially entered service with No.7 Squadron and at the peak of their service they equipped 13 RAF Bomber Command Squadrons (Nos.7, 15, 75, 90, 101, 149 166, 199, 214, 218, 513, 622 and 623). Starting in 1944 the main role of the Stirling was that of glider-tug and transport with RAF Transport Command. For D-Day on 6 June 1944 RAF Transport Command Squadrons Nos 190 and 622 from Fairford and Nos 196 and 299 from Keevil towed Airspeed Horsa gliders into Normandy. Late in the war, Squadron Nos 171, 295, 570, 620 and 624 also used the Stirling and participated in the airborne landings in Arnhem and the March 1945 attack across the Rhine. Squadron Nos 138 and 161 were (Special Duties) Squadrons, flying for the Special Operations Executive (SOE) from Tempsford, near Cambridge. They performed Covert Operations supplying arms to the Resistance in occupied countries. Based in Blida, North Africa, No. 624 Squadron performed the same task in the Mediterranean area.
Always remembered as the first of the four-engined bombers to join the RAF, the Stirling suffered from several design limitations which severely affected its performance and load-carrying capability. As a consequence, its service with Bomber Command was marred by heavy losses when used on operations alongside the higher-flying Halifaxes and Lancasters.
Air Ministry Specification B12/36, to which the Stirling was one of 11 designs proposed by various companies, called for a four-engined heavy bomber capable of carrying a bombload of 14,000lbs with a range of 3,000 miles a remarkably demanding request for the time). It also specified that the wingspan should not exceed 100 feet to enable the aircraft to fit inside current RAF hangars (although, curiously enough, the most common type of hangar, the C Type, could open to over 125 feet). As a consequence, certain aspects of the Stirling’s performance suffered namely that operating altitude of the aircraft with a full load, as the wings could not generate the lift required to operate a higher altitudes.
Shorts, the aircraft’s designers, were well versed in the design of flying boats and had never designed an aircraft with retracting undercarriage before. Taking the basic design of the company’s most recent flying boats, Shorts modified them to accommodate four engines and wheel undercarriage. To prove the design, Shorts built a half-scale prototype and this flew for the first time in September 1938 and after a series of test flights it was decided that the aircraft take-off and landing runs were overly long. This was countered by increasing the length of the undercarriage legs to increase the angle of the wings to the ground, but the legs were overly-complicated and lanky affairs and throughout its life, the Stirling suffered from a number of undercarriage-related accidents.
Early test flights of the Stirling were dramatic affairs; on the maiden flight of the first full-size prototype in May 1939, a wheelbrake locked on landing causing the aircraft to slew violently and collapse one of the undercarriage legs. The aircraft was a write-off. On the first flight of the second prototype two months later, and engine cut-out on take-off but the aircraft was landed safely.
Early aircraft had a retractable belly turret but this was soon discarded after a number of leaks had caused the turret to lower and strike the ground while the aircraft was taxying. With other minor problems cured, the Stirling finally flew its maiden operation during the night of 10th/11th February 1941 when aircraft from No 7 Squadron took part in a raid on Rotterdam. But then another fault with the aircraft’s design was encountered. As Bomber Command started operations in earnest over Germany towards the end of 1941, the lack of power produced by the four Bristol Taurus engines severely limited the loads carried by Stirlings. On missions against long-range targets such as Italy or deep inside Germany, the Stirling was restricted to 3,500lbs of bombs (seven 500lb-ers) and could barely climb over the Alps during the flights to and from the targets. The design of the bomb-bay meant that the heaviest bomb that could be carried was the 2,000lb armour-piercing shell – the new 4,000lb High Capacity bomb being introduced was too big for the compartmentalised bay of the Stirling.
Some way to remedying the poor performance of the basic Stirling design with the introduction of the Mark III from the start of 1943, but still the aircraft suffered much higher losses than the other aircraft of the Main Force. Within five months of being introduced, 67 out of the 84 aircraft delivered had been lost to enemy action or written off after crashes. During the year, the Stirlings were gradually phased out of the Main Force and moved to less dangerous duties such as minelaying. Only one Stirling squadron served with the Pathfinders – No 7 – but the Stirlings had been replaced by Lancasters by mid-1943.