The American atomic bomb dropped on Nagasaki on 9 August 1945 was called Fat Man.
Fat Man was a plutonium implosion weapon carried by the B-29 Bockscar, piloted by Major Charles Sweeney. It was released over Nagasaki shortly after 11 a.m. local time. The weapon detonated above the city, which contained important industrial facilities and a major port.
Fat Man was more technically complex than Little Boy, the uranium gun-type bomb used against Hiroshima three days earlier. The two weapons therefore differed in both design and nuclear material, even though both were developed through the Manhattan Project and used against Japan in the final weeks of World War II.
Nagasaki was not the original primary target for the mission. Kokura was listed first, but cloud and smoke obscured the aiming point, so the aircraft proceeded to its secondary target. The bombing occurred on the same day that Soviet forces began attacking Japanese-held Manchuria, adding to the crisis facing Japan’s government.