Fat Man " was the codename for the nuclear bomb that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945 . It was the second of the only two nuclear weapons ever used in warfare , the first being Little Boy , and its detonation marked the third nuclear explosion in history . It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site , and it was dropped from the Boeing B-29 Superfortress '' Bockscar '' piloted by Major Charles Sweeney . The name Fat Man refers to the early design of the bomb because it had a wide , round shape ; it was also known as the Mark III . Fat Man was an implosion-type nuclear weapon with a solid plutonium core . The first of that type to be detonated was the Gadget in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico . Two more were detonated during the Operation Crossroads nuclear tests at Bikini Atoll in 1946 , and some 120 were produced between 1947 and 1949 , when it was superseded by the Mark 4 nuclear bomb . The Fat Man was retired in 1950
Trinity nuclear test
Trinity (nuclear test)
Fat Man