Four Little Boy assemblies , L-1 , L-2 , L-5 and L-6 were expended in test drops . L-6 was used in the Iwo Jima dress rehearsal on 29 July . This was repeated on 31 July , but this time L-6 was test dropped near Tinian by '' Enola Gay '' . L-11 was the assembly used for the Hiroshima bomb . The Little Boy team had it 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 the President . 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 . " Assembly of a Fat Man unit was a complex operation involving personnel from the HE-ME , Pit , Fusing and Firing teams . To prevent the assembly building from becoming overcrowded and thereby causing an accident , Parsons limited the numbers allowed inside at any time . Personnel waiting to perform a specific task had to wait their turn outside the building . The first Fat Man assembly , known as F13 , was assembled by 31 July , and expended in a drop test the next day . This was followed by F18 on 4 August , which was dropped the next day . Three sets of Fat Man high explosive pre-assemblies , designated F31 , F32 , and F33 , arrived on a B-29 of the 509th Composite Group and 216th Army Air Forces Base Unit on 2 August . On inspection , the high explosive blocks of F32 were found to be badly cracked and unserviceable . The other two were assembled , with F33 earmarked for a rehearsal and F31 for operational use
Potsdam Conference
Potsdam Conference
Project Alberta