Fat Man was the United States’ codename for the atomic bomb dropped on Nagasaki in 1945.
The weapon was a plutonium-fueled implosion-type bomb developed by the Manhattan Project. It was dropped from the B-29 Superfortress Bockscar on August 9, 1945, and detonated over Nagasaki shortly after 11 a.m. local time.
Fat Man differed technically from Little Boy, the uranium gun-type bomb used against Hiroshima three days earlier. The two weapons used different nuclear materials and different internal designs. Fat Man was wider and more rounded, which inspired its name.
The Nagasaki mission was intended to demonstrate that the United States possessed another atomic weapon after Hiroshima. The bombings, combined with the Soviet declaration of war and invasion of Manchuria, intensified pressure on Japan’s leadership. Japan announced its acceptance of the Potsdam Declaration on August 15, while the legal surrender was signed on September 2.