In 1942, U.S. President Roosevelt authorized a secret programme to create an atomic bomb. Major General Leslie R. Groves was put in charge with physicist J. Robert Oppenheimer designing and building the bomb in a Laboratory, code-named Project Y.

Project Y scientists began the development of two bombs – one using uranium and one using plutonium. In the Spring of 1944, the scientists found that a plutonium bomb would not work – it would blow itself apart before the two pieces of plutonium could fully unite to create the supercritical mass required for a successful explosion.

Confronted with this problem, Oppenheimer requested the team pursue the development of a plutonium bomb based on the concept of implosion: conventional explosives would be detonated around a sub-critical core of plutonium, compressing it into a supercritical mass.

On July 16, 1945, members of Project Y and other observers gathered at a remote location in New Mexico some 200 miles south of Los Alamos, near Alamogordo. Some three weeks later, a U.S. Army Air Force B-29 dropped a uranium bomb known as Little Boy (named after Roosevelt) over the Japanese City of Hiroshima. Three days later, another B-29 dropped a plutonium bomb know as Fat Boy (named after Winston Churchill) over Nagasaki. Detonated in the air, the bombs caused immense destruction and killed well over a hundred thousands people.

Two days after Fat Boy was detonated, Japan agreed to surrender and World War II came to an end.

A replica of the Fat Boy atomic bomb shell at the Bradbury Science Museum in Los Alamos, New Mexico.