Groves Jr., who in turn assembled a special committee consisting of Lawrence, Compton, Oppenheimer, and McMillan to examine the issue. Conant informed Manhattan Project director Brigadier General Leslie R. Conant consulted Ernest Lawrence and Arthur Compton, who acknowledged that their scientists at Berkeley and Chicago respectively knew about the problem, but they could offer no ready solution. Wallace Akers, the director of the British ' Tube Alloys' project, told James Bryant Conant on 14 November that James Chadwick had 'concluded that plutonium might not be a practical fissionable material for weapons because of impurities'. The feasibility of a plutonium bomb was questioned in 1942. Tolman suggested an implosion-type nuclear weapon, but the proposal attracted little interest. They chose a gun-type design in which two sub-critical masses would be brought together by firing a 'bullet' into a 'target'. Robert Oppenheimer held conferences in Chicago in June 1942, prior to the Army taking over wartime atomic research, and in Berkeley, California, in July, at which various engineers and physicists discussed nuclear bomb design issues. Two more were detonated during the Operation Crossroads nuclear tests at Bikini Atoll in 1946, and some 120 were produced between 19, when it was superseded by the Mark 4 nuclear bomb. The first of that type to be detonated was the Gadget in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico. Fat Man was an implosion-type nuclear weapon with a solid plutonium core.

The name Fat Man refers to the early design of the bomb because it had a wide, round shape.

It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site, and was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney. It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation marked the third nuclear explosion in history. ' Fat Man' (also known as Mark III) was the codename for the type of nuclear bomb the United States detonated over the Japanese city of Nagasaki on 9 August 1945.

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