Day bomber

While bombing was usually performed by the crews of single-engine aircraft, the heavy bomber concept was pioneered by the three-engine Caproni Ca.1 of the Italian Corpo Aeronautico Militare and the four-engine Sikorsky Ilya Muromets, operated by the Russian Imperial Military Aviation Fleet.

The DH.4 entered service with the British Royal Flying Corps in early 1917, and was regarded as a successful design: it served with the militaries of at least 14 countries and 6,295 were built in total (most of them under license in the United States).

Undertaking daylight raids in the early stages of World War II, even the most effective bombers were found to suffer heavy losses and aircrew casualties, as a result of fighter attacks.

After the United States entered the war, its main heavy bombers at the time – such the Boeing B-17 Flying Fortress and Consolidated B-24 Liberator – were seen to be so well-armed that they were suitable for unescorted daylight missions.

At the same time, general purpose bombers and multirole aircraft were becoming more and more prominent: such designs were often capable of relatively high top speeds and rapid acceleration, assisting their pilots in avoiding direct attacks from fighters.