 Thin Man as a plutonium @-@ based design was therefore abandoned and the weapon was re @-@ designed to use uranium 235 . The muzzle velocity required was much lower , reducing the barrel length of the resulting bomb , now code @-@ named Little Boy , to less than 10 feet ( 3 @.@ 0 m ) . This allowed the device to fit into a standard B @-@ 29 bomb bay , so the Pullman was modified to its original configuration with the rear bomb bay a standard B @-@ 29 design . All subsequent Silverplates were also configured in this manner . The Pullman B @-@ 29 was flown to Wendover Army Air Field in Utah in September 1944 . It carried out further drop testing with the 216th Army Air Forces Base Unit until it was damaged in a landing accident in December . 
