Although not as suitable for the atomic mission as the British Avro Lancaster with its cavernous 33 feet bomb bay, Major General Leslie R. Groves, Jr., the director of the Manhattan Project, and General Henry H. Arnold, the Chief of United States Army Air Forces (USAAF), wanted to use an American plane, if this was at all possible, so the Boeing B-29 Superfortress was chosen, even though it required substantial modification.