Fat Man was the American code name for the atomic bomb dropped on Nagasaki.
The weapon was a plutonium-239 implosion bomb that detonated over Nagasaki on August 9, 1945. It was carried by the B-29 Bockscar, piloted by Major Charles Sweeney. The bomb had a rounded, bulbous shape, which inspired its code name.
Fat Man differed technically from Little Boy, the uranium gun-type bomb dropped on Hiroshima three days earlier. The plutonium design required a much more complex arrangement of conventional explosive lenses to compress the nuclear material and begin the chain reaction.
The Nagasaki mission originally aimed at Kokura, but cloud and smoke blocked a clear view of that target. Bockscar diverted to Nagasaki and released the bomb after finding a limited opening in the weather. Although the bombing contributed to the crisis leading to surrender, Japan’s decision also followed the Soviet declaration of war and invasion of Manchuria.