 Prior to the decision to use the B @-@ 29 , serious consideration had been given to using the British Avro Lancaster with its cavernous 33 @-@ foot ( 10 m ) bomb bay to deliver the weapon . This would have required much less modification , but would have required additional crew training for the USAAF crews . Major General Leslie R. Groves , Jr . , the director of the Manhattan Project , and General Henry H. Arnold , the Chief of United States Army Air Forces ( USAAF ) , wished to use an American plane , if this was at all possible . 
