 The Mark III Mod 0 Fat Man was ordered to be put into production in mid @-@ 1946 . High explosives were manufactured by the Salt Wells Pilot Plant , which had been established by the Manhattan Project as part of Project Camel . A new plant was established at the Iowa Army Ordnance Plant . Mechanical components were made or procured by the Rock Island Arsenal . Electrical and mechanical components for about 50 bombs were stockpiled at Kirtland Army Air Field by August 1946 , but only nine plutonium cores were available . Production of the Mod 0 ended in December 1948 , by which time there were still only 53 cores available . It was replaced by improved versions , known as Mods 1 and 2 , which contained a number of minor changes , the most important of which was that they did not charge the X @-@ Unit firing system 's capacitors until released from the aircraft . The Mod 0s were withdrawn from service between March and July 1949 , and by October they had all been rebuilt as Mods 1 and 2 . Some 120 Mark III Fat Man units were added to the stockpile between 1947 and 1949 , when it was superseded by the Mark 4 nuclear bomb . The Mark III Fat Man was retired in 1950 . 
