The Boeing B-29 Superfortress was the most expensive U.S. weapons program of World War II, its cost of US$ 3 billion in then-dollars (around US$ 45 billion today) more than 50% greater than the US$ 1.9 billion Manhattan project. It was designed for long-range, high-altitude, precision strategic bombing, allowing raids upon the Japanese home islands from island bases and China, which the B-17s used in the European theatre lacked the range to perform.
Development of the B-29 and its Wright R-3350 Duplex-Cyclone engines was fraught with problems, and those problems, particularly engine fires and other failures, persisted after its entry into service. In the campaign against Japan, a total of 414 B-29s were lost, 147 were downed by enemy action, while 267 were lost due to operational problems, mostly relating to the engines.
The B-29 performed poorly in its intended role as a high altitude bomber. It was only when Curtis LeMay changed the strategy against Japan to night raids with incendiary bombs that the B-29 became effective. But this was a mission for which it was not designed, and only possible because by that time most of Japan’s defensive fighter forces had been eliminated. In early 1945, LeMay ordered most of the defensive armament of the B-29 removed in order to increase the bomb load, so this complicated, expensive, and heavy component of the B-29 was little used.
It was only after the end of the war, when the B-29D/B-50 version replaced the Wright engines with the Pratt and Whitney R-4360 Wasp Major engine that the model became reliable and safe to operate.