Because of advances in aircraft design and engineering — especially in powerplants and aerodynamics — the size of payloads carried by heavy bombers have increased at rates greater than increases in the size of their airframes. The largest bombers of World War I, the four engine aircraft built by the Zeppelin-Staaken company in Germany, could carry a payload of up to 4,400 pounds (2,000 kg) of bombs. By the middle of World War II even a single-engine fighter-bomber could carry a 2,000-pound (910 kg) bomb load, and such aircraft were taking over from light and medium bombers in the tactical bombing role. Advancements in four-engine aircraft design enabled heavy bombers to carry even larger payloads to targets thousands of kilometres away. For instance, the Avro Lancaster (introduced in 1942) routinely delivered payloads of 14,000 pounds (6,400 kg) (and sometimes up to 22,000 lb/10,000 kg) and had a range of 2,530 miles (4,070 km). The B-29 (1944) delivered payloads in excess of 20,000 pounds (9,100 kg) and had a range of 3,250 miles (5,230 km). By the early 1960s, the jet-powered Boeing B-52 Stratofortress, travelling at speeds of up to 650 miles per hour (1,050 km/h) (i.e., more than double that of a Lancaster), could deliver a payload of 70,000 pounds (32,000 kg), over a combat radius of 4,480 miles (7,210 km).
During World War II, mass production techniques made available large, long-range heavy bombers in such quantities as to allow strategic bombing campaigns to be developed and employed. This culminated in August 1945, when B-29s of the United States Army Air Forces dropped atomic bombs over Hiroshima and Nagasaki in Japan, contributing significantly to the end of hostilities.
The arrival of nuclear weapons and guided missiles permanently changed the nature of military aviation and strategy. After the 1950s intercontinental ballistic missiles and ballistic missile submarines began to supersede heavy bombers in the strategic nuclear role. Along with the emergence of more accurate precision-guided munitions ("smart bombs") and nuclear-armed missiles, which could be carried and delivered by smaller aircraft, these technological advancements eclipsed the heavy bomber's once-central role in strategic warfare by the late 20th century. Heavy bombers have, nevertheless, been used to deliver conventional weapons in several regional conflicts since World War II (e.g., B-52s in the Vietnam War).
Heavy bombers are now operated only by the air forces of the United States, Russia and China. They serve in both strategic and tactical bombing roles.
The first heavy bomber was designed as an airliner. Igor Sikorsky, an engineer educated in St Petersburg, but born in Kiev of Polish-Russian ancestry designed the Sikorsky Ilya Muromets to fly between his birthplace and his new home. It did so briefly until August 1914, when the Russo-Balt wagon factory converted to a bomber version, with British Sunbeam Crusader V8 engines in place of the German ones in the passenger plane. By December 1914 a squadron of 10 was bombing German positions on the Eastern Front and by summer 1916 there were twenty. It was well-armed with nine machine guns, including a tail gun and initially was immune to German and Austro-Hungarian air attack. The Sikorsky bomber had a wingspan just a few feet shorter than, with a bomb load only 3% of, a World War II Avro Lancaster.