 In October 1951 , in anticipation of a major build @-@ up of the USAF to 95 groups , and of delays in the B @-@ 47 program , the Air Materiel Command was ordered to modify a further 180 B @-@ 29s then in storage to carry the Mark 4 , Mark 5 , Mark 6 , and Mark 8 nuclear bombs . Modifications were carried out in Oklahoma City and Sacramento . By the time the aircraft were all delivered in September 1953 , the B @-@ 29 was being phased out , and the modified aircraft went back into storage without seeing further service . 
