 The size of the bomb was constrained by the available aircraft . The only Allied aircraft capable of carrying the Fat Man were the British Avro Lancaster and the American Boeing B @-@ 29 Superfortress . For logistic and nationalistic reasons , the B @-@ 29 was preferred , but this constrained the bomb to a maximum length of 132 inches ( 3 @,@ 400 mm ) , width of 60 inches ( 1 @,@ 500 mm ) and weight of 20 @,@ 000 pounds ( 9 @,@ 100 kg ) . Removing the bomb rails allowed a maximum width of 66 inches ( 1 @,@ 700 mm ) . Drop tests began in March 1944 , and resulted in modifications to the Silverplate aircraft due to the weight of the bomb . High @-@ speed photographs revealed that the tail fins folded under the pressure , resulting in an erratic descent . Various combinations of stabilizer boxes and fins were tested on the Fat Man shape to eliminate its persistent wobble until an arrangement dubbed a " California Parachute " , a cubical open @-@ rear tail box outer surface with eight radial fins inside of it , four angled at 45 ° and four orthogonally to the line of fall holding the outer square @-@ fin box to the bomb 's rear end , was approved . In drop tests in early weeks , the Fat Man missed its target by an average of 1 @,@ 857 feet ( 566 m ) , but this was halved by June as the bombardiers became more proficient with it . 
