 Groves met with Arnold again in September 1943 . He informed Arnold that there was now a second bomb shape under consideration , the Fat Man , and formally requested that further tests be carried out , that not more than three B @-@ 29s be modified to carry the weapons , and that the USAAF form and train a special unit to deliver the bombs . Arnold delegated responsibility for this to Major General Oliver P. Echols . In turn , Echols designated Colonel Roscoe C. Wilson as the Project Officer . 
