Project Alberta proceeded with the plan to have the Little Boy ready by 1 August , and the first Fat Man ready for use as soon as possible after that . In the meantime , a series of twelve combat missions were flown between 20 and 29 July against targets in Japan using high-explosive pumpkin bombs , versions of the Fat Man with the explosives , but not the fissile core . Project Alberta 's Sheldon Dike and Milo Bolstead flew on some of these missions , as did the British observer Group Captain Leonard Cheshire . Four Little Boy pre-assemblies , L-1 , L-2 , L-5 and L-6 were expended in test drops . The Little Boy team had the live bomb completely assembled and ready for use on 31 July . The final item of preparation for the operation came on 29 July 1945 . Orders for the attack were issued to General Carl Spaatz on 25 July under the signature of General Thomas T. Handy , the acting Chief of Staff of the United States Army , since General of the Army George C. Marshall was at the Potsdam Conference with President Harry S. Truman . The order designated four targets : Hiroshima , Kokura , Niigata , and Nagasaki , and ordered the attack to be made " as soon as weather will permit after about 3 August "
Potsdam Conference
Potsdam Conference
Project Y