Fat Man was the name of the second atomic bomb dropped on Japan in August 1945.
The United States dropped Fat Man over Nagasaki on August 9, 1945. It was an implosion-type plutonium weapon, unlike Little Boy, the uranium gun-type bomb dropped over Hiroshima three days earlier. The attack was carried out by the B-29 bomber Bockscar.
The original target was Kokura, but cloud cover and smoke prevented the crew from making a visual bombing run. The aircraft diverted to Nagasaki, where a break in the clouds allowed the bomb to be released. Terrain limited some of the blast's effects, but the attack still caused immense destruction and loss of life.
Fat Man and Little Boy were code names rather than descriptions used by Japan. The names belonged to the U.S. wartime atomic-bomb program. Japan announced its acceptance of the Allied surrender terms less than a week after the Nagasaki bombing.