 " Fat Man " was the codename for the type of atomic 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 @-@ ever man @-@ made nuclear explosion in history . It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site and dropped from the Boeing B @-@ 29 Superfortress Bockscar . For the Fat Man mission , Bockscar was piloted by Major Charles W. Sweeney . 
