Little Boy

Little Boy
A post-war Little Boy model
Type	Nuclear weapon
Place of origin	United States
Production history
Designer	Los Alamos Laboratory
Manufacturer	Naval Gun Factory, Washington, D.C. Naval Ordnance Plant, Center Line, Michigan Expert Tool and Die Company, Detroit, Michigan
Produced	1945–1947
No. built	1 wartime + 5 postwar
Specifications
Mass	9,700 pounds (4,400 kg)
Length	10 feet (3.0 m)
Diameter	28 inches (71 cm)
Filling	Highly enriched uranium
Filling weight	64 kg (141 lb)
Blast yield	15 kilotons of TNT (63 TJ)

Little Boy was a type of atomic bomb created by the United States as part of the Manhattan Project during World War II. The name is also often used to describe the specific bomb (L-11) used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress Enola Gay on 6 August 1945, making it the first nuclear weapon used in warfare, and the second nuclear explosion in history, after the Trinity nuclear test. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and had an explosion radius of approximately 1.3 kilometres (0.81 mi) which caused widespread death across the city. It was a gun-type fission weapon which used uranium that had been enriched in the isotope uranium-235 to power its explosive reaction.

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Los Alamos Laboratory. It was the successor to a plutonium-fueled gun-type fission design, Thin Man, which was abandoned in 1944 after technical difficulties were discovered. Little Boy used a charge of nitrocellulose to fire a hollow cylinder (the "bullet") of highly enriched uranium through an artillery gun barrel into a solid cylinder (the "target") of the same material. The design was highly inefficient: the weapon used on Hiroshima contained 64 kilograms (141 lb) of uranium, but less than a kilogram underwent nuclear fission. Unlike the implosion design developed for the Trinity test and the Fat Man bomb design that was used against Nagasaki, which required sophisticated coordination of shaped explosive charges, the simpler but inefficient gun-type design was considered almost certain to work, and was never tested prior to its use at Hiroshima.

After the war, numerous components for additional Little Boy bombs were built. By 1950, at least five weapons were completed; all were retired by November 1950.