Fat Man was the code name of the second atomic bomb dropped on Japan before its surrender.
The United States dropped Fat Man on Nagasaki on August 9, 1945, three days after Little Boy was dropped on Hiroshima. Fat Man used a plutonium implosion design, whereas Little Boy used a uranium gun-type design. The two weapons therefore differed substantially in engineering as well as in code name.
The B-29 bomber Bockscar carried Fat Man. Nagasaki was not the aircraft’s original primary target: Kokura was listed first, but cloud cover and smoke obscured the aiming point. The crew then proceeded to Nagasaki, where a break in the clouds allowed the bomb to be released.
Japan announced its surrender six days later, on August 15. The effects of the bombings, the Soviet entry into the war, and Japan’s internal political crisis all form part of the historical debate about why the surrender decision was made.