The Boeing B-29 Superfortress entered service in 1944 and was, by a considerable margin, the most technically advanced aircraft of the Second World War. It was also the most expensive American programme of the war — costing more than the Manhattan Project.
What was new about it
Pressurisation. The B-29 had a pressurised crew compartment, which meant crews could operate at 30,000 feet without oxygen masks and heated suits. Because the bomb bay could not be pressurised, the forward and rear compartments were connected by a sealed tunnel over it, which crew crawled through.
Remote gun turrets. Rather than gunners sitting in each turret, the B-29 used a central fire control system. Gunners at sighting stations aimed through computing gunsights, and an analogue computer corrected for parallax, lead and ballistics, then aimed the remote turrets. One gunner could control several turrets.
Range and load. It could carry a large bomb load across the Pacific distances the war in Japan required, which no other Allied bomber could do.
The development problems
The programme was rushed, and the Wright R-3350 engines were the weak point. They overheated badly, and magnesium components in the engine could catch fire and burn hot enough to sever a wing spar.
Engine fires killed test crews, including on a prototype that crashed into a Seattle factory in 1943. Early operational units lost more aircraft to mechanical failure than to enemy action.
Modifications eventually made the engine acceptable, but it required constant attention throughout the aircraft's service life.
The campaign over Japan
The B-29 was designed for high-altitude precision bombing. Over Japan that did not work: the jet stream at 30,000 feet produced winds strong enough to make accurate bombing nearly impossible, and cloud cover was persistent.
General Curtis LeMay changed the tactics fundamentally in early 1945 — low altitude, at night, with incendiaries, and with defensive guns removed to carry more bombs. The firebombing raids on Japanese cities that followed killed very large numbers of civilians; the raid on Tokyo of 9-10 March 1945 is estimated to have killed around 100,000 people in a single night.
Two B-29s, Enola Gay and Bockscar, dropped the atomic bombs on Hiroshima and Nagasaki in August 1945.
The Soviet copy
Three B-29s that force-landed in the Soviet Union were not returned. Stalin ordered them reverse-engineered, and the Tupolev Tu-4 was the result — a near-exact copy, reportedly reproducing even manufacturing details that served no purpose in the original.
The Tu-4 gave the Soviet Union its first intercontinental-capable bomber and shaped early Cold War strategic planning.
Afterwards
B-29s served in Korea and were retired in the 1960s. Nearly 4,000 were built.
Two remain airworthy, both in the United States, maintained at enormous expense — the R-3350 remains as difficult as it ever was.