Fat Man was the second atomic bomb used against Japan in World War II.
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, while Little Boy used a gun-type uranium design. Both weapons were developed through the Manhattan Project.
The Nagasaki mission was complicated by weather and fuel limitations. The aircraft, Bockscar, dropped the bomb over Nagasaki after the primary target, Kokura, was obscured by cloud and smoke. The explosion caused catastrophic destruction and loss of life.
Fat Man is sometimes confused with Trinity, the first nuclear device tested by the United States. Trinity was a test device detonated in New Mexico on July 16, 1945, not a bomb dropped on a Japanese city.