 The Los Alamos Laboratory and the Army Air Forces had already commenced work on improving the design . The North American B @-@ 45 Tornado , Convair XB @-@ 46 , Martin XB @-@ 48 , and Boeing B @-@ 47 Stratojet bombers , then on the drawing boards , had bomb bays sized to carry the Grand Slam , which was much longer but not as wide as the Fat Man . The only bombers that could carry the Fat Man were the B @-@ 29 and the Convair B @-@ 36 . In November 1945 , the Army Air Forces asked Los Alamos for 200 Fat Man bombs . At the time there were only two sets of plutonium cores and high @-@ explosive assemblies . The Army Air Forces wanted improvements to the design to make it easier to manufacture , assemble , handle , transport , and stockpile . The wartime Project W @-@ 47 was continued , and drop tests resumed in January 1946 . 
