A change of engine from the 708 kW (950 hp) M-87B to the 820 kW (1,100 hp) M-88 resulted in the DB-3F, which were eventually renamed in 1942 as the Il-4. 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 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.
Although the Il-4 was only a medium bomber, it had the range to be used on strategic missions. The VVS wasn't terribly interested in this role, but nevertheless the Il-4 was used on several highly publicized raids against Berlin. Most would be used on much shorter range missions, often adding another 1,000 kg (2,204 lb) of bombs under the wings, in addition to the internal 2,500 kg (5,512 lb).
The Il-4 remained in production until 1944, when just over 5,200 had been built.
