The $3 billion cost of design and production of the B-29 Superfortress (equivalent to $43 billion today) far exceeded the $1.9 billion cost of the Manhattan Project and made the B-29 program the most expensive of the war.
Boeing B-29 Superfortress
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One of the largest aircraft of World War II, the B-29 had state-of-the-art technology, including a pressurized cabin, dual-wheeled, tricycle landing gear, and an analog computer-controlled fire-control system that allowed one gunner and a fire-control officer to direct four remote machine gun turrets.