The bomb dropped on Nagasaki on 9 August 1945 was code-named Fat Man.
Fat Man was an American plutonium-implosion atomic bomb developed at the Los Alamos Laboratory. It was carried by the B-29 bomber Bockscar and detonated over Nagasaki at 11:02 a.m. local time. The aircraft had originally approached Kokura, but clouds and smoke obscured that primary target, so the crew proceeded to Nagasaki.
Fat Man was the second nuclear weapon used in warfare, following Little Boy, which was dropped on Hiroshima three days earlier. The two weapons used different designs: Little Boy used enriched uranium in a gun-type mechanism, while Fat Man used plutonium in an implosion design.
A common mix-up is calling both bombs Fat Man or Little Boy. Little Boy was used at Hiroshima; Fat Man was used at Nagasaki. The Nagasaki bombing, together with the Soviet declaration of war, preceded Japan’s surrender announcement six days later.