In addition Twentieth Air Force was chosen (secretly) to be the operational component of the Manhattan Project in 1944, and performed the atomic attacks on Japan in August 1945. However, in early 1944, the B-29 was not yet operationally ready. The aircraft had been in development at Boeing since the late 1930s and the first XB-29 (41-0002) flew on 21 September 1942. However, the aircraft suffered from an overwhelming number of development issues, and with engine problems (fires). As a result, most of the first production B-29s were still held up at Air Technical Service Command modification centers, awaiting modifications and conversion to full combat readiness. By March 1944, the B-29 modification program had fallen into complete chaos, with absolutely no bombers being considered as combat ready. The program was seriously hampered by the need to work in the open air in inclement weather, as many hangars were simply too small to house the aircraft indoors; by delays in acquiring the necessary tools and support equipment, and by the USAAF's general lack of experience with the B-29.