The term was used prior to and during World War II, when available engine power for bomber aircraft designs had to be carefully tailored to their missions.
The medium bomber was generally considered to be any level bomber design that delivered about 4,000 pounds (1,800 kg) of ordnance over ranges of about 1,500 to 2,000 mi (2,400 to 3,200 km). Heavy bombers were those with a nominal load of 8,000 pounds (3,600 kg) or more, and light bombers carried 2,000 lb (907 kg). These distinctions were beginning to disappear by the middle of World War II, when the average fighter aircraft could now carry a 2,000 lb (907 kg) load and ever more powerful engines allowed "light" bomber designs (and later jetfighter-bombers) to largely take over the missions formerly filled by mediums.
After the war the term disappeared from use almost instantly; some of this was due to mass demobilization of the participant air forces' existing equipment, and the fact that several of the most-produced medium bomber types were now technologically obsolescent. Although a number of later aircraft were designed in this performance and load-carrying range, they were henceforth referred to as tactical bombers instead. Examples of post-war mediums include the English Electric Canberra (along with its derived U.S. counterpart, the Martin B-57) and the Soviet Ilyushin Il-28 "Beagle".