Design and Development
In 1938, the Ilyushin design bureau redesigned the Ilyushin DB-3 to ease production and improve performance, the revised version receiving the designation DB-3F (Forsirovannyi or "boosted"). The aircraft's internal structure, particularly the wings, was extensively changed, eliminating the need for hand finishing of the structure and with Duralumin replacing the large scale use of steel in the earlier aircraft. The aircraft's fuel system was redesigned, increasing capacity while reducing the number of fuel tanks. The fuselage nose was lengthened to give more room for the navigator/bomb-aimer while reducing drag. The prototype DB-3F, powered by the same 708 kW (949 hp) Tumansky M-87B engines of the DB-3M, was piloted on its maiden flight by Vladimir Kokkinaki on 21 May 1939. It successfully passed through state acceptance tests and entered production in January 1940, with the 1,100 hp (820 kW) Tumansky M-88 quickly replacing the M-87. The DB-3F was redesignated Il-4 in March 1942. Some series had wooden outer wings and front fuselage to conserve metals, and throughout the production engines and fuel tanks were upgraded for more performance while keeping the same range. However the most notable change was the addition of larger defensive guns in the turret, using the 12.7 mm (0.5 in) UBT machine gun in place of the earlier 7.62 mm (0.3 in) weapons. In addition it was found that the gunners were attacked first, so blocks of armor were placed around the gunner positions.
All this extra weight wasn't offset by the newer engines however, and the Il-4 proved to be slower than the earlier versions at only 404 km/h (251 mph). An attempt to improve performance was made as the Il-6, adding large diesel engines and heavier armament. The engine proved unreliable and production was never started. The Il-4 remained in production until 1944, when just over 5,200 had been built.
Read more about this topic: Ilyushin Il-4
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