The Stirling was the first four-engined British heavy bomber of the Second World War. Built by Short Brothers, it was to have a relatively short operational career. Throughout the 1930s, the Royal Air Force was interested primarily in twin-engine bombers and invested heavily in development of huge engines in the 2,000 horsepower (1,500 kW) class in order to improve performance. In the U.S. and USSR were developing bombers with four smaller engines, which proved to have excellent range and fair lifting capacity, so in 1936 the RAF also decided to investigate the feasibility of the four-engined bomber. The Air Ministry Specification B.12/36 had a mixture of requirements.[citation needed] In addition to a 14,000 lb (6,350 kg) bombload carried to a range of 3,000 miles (4,800 km) (incredibly demanding for the era), the aircraft should also be able to be used as a troop transport for 24 soldiers. The idea was that it 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. Since it could be operating from limited "back country" airfields, it needed to lift off from a 500 ft (150 m) runway and able to clear 50 ft (15 m) trees at the end, a specification most small aircraft would have a problem with today. The wingspan was limited to 100 ft (30 m) so the aircraft would fit into existing hangars. The wingspan limit was also imposed in an unsuccessful attempt to ensure the Stirling's weight was kept down. Operational status wasn't reached until January 1941 by No. 7 Squadron RAF. The first three Stirlings flew a mission on 10 February 1941 over fuel storage tanks in Rotterdam, and from spring of 1942 it started to be used in greater numbers. From May 1943, air raids on Germany started with over a hundred Stirlings at once. Despite the "disappointing performance" at maximum altitude, Stirling pilots were delighted to discover that, due to the thick wing, they could out-turn the Ju 88 and Me 110 nightfighters they faced. Its handling was much better than that of the Halifax and some preferred it to the Lancaster. Based on its flight characteristics, Pilot Murray Peden of No. 214 RAF Squadron flatly described the Stirling as "one of the finest aircraft ever built." Another consequence of the thick wing however was a low ceiling and many missions were flown as low as 12,000 ft (4,000 m). This was a disadvantage on many raids, notably if crews were attacking Italy and had to fly through (rather than "over") the Alps. When Stirlings were on combined operations with other RAF bombers which could fly at higher altitudes, the Luftwaffe concentrated on the low-flying Stirlings. Within five months of being introduced, 67 out of the 84 aircraft delivered had been lost to enemy action or written off after crashes. The Stirling's huge maximum bomb load was only able to be carried for relatively short distances of around 590 miles. On typical missions deep into Germany or Italy a smaller 3,500 lb (1,590 kg) load was carried, consisting of seven 500 lb (227 kg) bombs. This was the sort of load being carried by the RAF's medium bombers such as the Vickers Wellington and, by 1944, by the de Havilland Mosquito. Perhaps the biggest problem with the design was that the bomb bay had two structural dividers running down the middle, limiting it to carrying nothing larger than the 2,000 lb (907 kg) bomb. As the RAF started using the 4000 lb (1,815 kg) "cookies" and even larger "specials," the Stirling became less useful. In 1943, it was decided to withdraw Stirlings to secondary tasks. By December 1943, Stirlings were being withdrawn from frontline service as a bomber, increasingly being used for deploying mines outside German ports, electronic countermeasures and dropping spies deep behind enemy lines at night (through the now unused ventral turret ring). General characteristics Crew: 7 Length: 87 ft 3 in (26.6 m) Wingspan: 99 ft 1 in (30.2 m) Height: 28 ft 10 in (8.8 m) Wing area: 1,322 ft² (122.8 m²) Empty weight: 44,000 lb (19,950 kg) Loaded weight: 59,400 lb (26,940 kg) Max takeoff weight: 70,000 lb (31,750 kg) Powerplant: 4× Bristol Hercules II radial engines, 1,375 hp (1,030 kW) each Performance Maximum speed: 255 mph (410 km/h) at 21,000 ft (6,400 m) Range: 2,330 mi (3,750 km) Service ceiling: 16,500 ft (5,030 m) Rate of climb: 800 ft/min (4 m/s) Wing loading: 44.9 lb/ft² (219.4 kg/m²) Power/mass: 0.093 hp/lb (0.153 kW/kg) Armament 8 x 0.303 in (7.7 mm) Browning machine guns: 2 in the nose, 4 in the tail, 2 dorsal Up to 18,000 lb (8,164 kg) of bombs