"Fat Man" is the code name for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945, at 11:02 a.m. (JSP). It was the second of the only two nuclear weapons to be used in warfare and was the third man-made nuclear explosion. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core. 
Type Nuclear weapon Place of origin United States Specifications Weight 10,200 lbs (4,630 kg) Length 10.6 Feet (3.25m) Diameter 5 Feet (1.52m) Blast yield 21 kilotons
Fat Man was possibly named after Winston Churchill , though Robert Serber said in his memoirs that as the "Fat Man" bomb was round and fat, he named it after Sydney Greenstreet's character of "Kasper Gutman" in The Maltese Falcon. The design of "Fat Man" nuclear assembly was substantially the same as "the gadget" detonated at the Trinity test in July 1945.
"Fat Man" was detonated at an altitude of about 1,800 feet (550 m) over the city, and was dropped from a B-29 bomber Bockscar, piloted by Major Charles Sweeney of the 393d Bombardment Squadron, Heavy. The bomb had a yield of about 21 kilotons of TNT, or 8.78×1013 joules = 88 TJ (terajoules). Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 39,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured. Thousands more would die later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiations. The aerial bombing raid on Nagasaki had the third highest fatality rate in World War II after the nuclear strike on Hiroshima and the March 9/10 1945 fire bombing raid on Tokyo.