|Working in collaboration with the United Kingdom and Canada, with their respective projects Tube Alloys and Chalk River Laboratories, the Manhattan Project, under the direction of Major General General Leslie Groves, of the U.S. Army Corps of Engineers, designed and built the first atomic bombs. Preliminary research began in 1939, originally in fear that the Nazi atomic bomb project would develop atomic weapons first. In May 1945, the defeat of Germany caused the focus to turn to use against Japan.
Two types of bombs were eventually devised by scientists and technicians at Los Alamos under American physicist Robert Oppenheimer. The Hiroshima bomb, known as Little Boy, was a gun-type fission weapon made with uranium-235, a rare isotope of uranium extracted in giant factories in Oak Ridge, Tennessee. The other was an implosion-type nuclear weapon using plutonium-239, a synthetic element created in nuclear reactors at Hanford, Washington. A test implosion weapon, the gadget, was detonated at Trinity Site, on 16 July 1945, near Alamogordo, New Mexico. The Nagasaki bomb, Fat Man was also an implosion device
Little Boy explodes over Hiroshima, Japan, 6 August 1945 (left);
Fat Man explodes over Nagasaki, Japan, 9 August 1945 (right).