The SAAB 21 is a Swedish single-seat low-wing monoplane fighter and attack aircraft designed and manufactured by SAAB. It used a relatively unorthodox twin boom fuselage with a pusher engine, giving the aircraft an unusual appearance. Work began at SAAB following a Swedish Air Force decision to embark on a major expansion programme in preparation for the possibility of being drawn into the Second World War. The company designed a monoplane twin-boom aircraft, powered by a single Daimler-Benz DB 605B engine that was positioned to the rear of the fuselage nacelle, directly behind the pilot, and drove a pusher propeller. This arrangement allowed guns to be carried in the aircraft's nose while providing the pilot with good visibility. To enable the pilot to bail out without hitting the propeller behind him, they adopted an ejection seat. On 30 July 1943, the 21 performed its maiden flight and on 1 December 1945, the first examples of the J 21A-1 were introduced to service. It was quickly followed by the improved J 21A-2, which featured heavier armament, and the A 21A-3 fighter-bomber. Due to Swedish Air Force interest in jet fighters, SAAB produced a conversion using the British de Havilland Goblin as the Saab 21R. The 21 was replaced in the mid-1950s after less than 10 years of service by the similarly configured de Havilland Vampire and the Saab 29 Tunnan. Design and development Background SAAB was carrying out design studies during the late 1930s into possible options for a new fighter aircraft. Many of these had been based around the use of a British Bristol Taurus radial engine and some were unconventional for the time. One of the configurations studied was of a monoplane pusher configuration twin-boom aircraft, with the engine behind the pilot at the rear of a central nacelle. This unorthodox design possessed several advantages, such as the ability to concentrate most of the guns in the aircraft's nose, good pilot visibility, and ease of service. While this promising design study was completed, it remained dormant until 1941, when defence considerations heightened the imminent need for it. At the start of Second World War, the Swedes became concerned about maintaining their neutrality and independence as they would soon be threatened by one or more European nations. As an emergency measure, the Swedish Air Force embarked upon a major rearmament and expansion of their military during the 1939–1941 period, which