The Fat Man plutonium implosion-type weapon was developed in a concerted design and development effort by the Los Alamos Laboratory.
After the feasibility of the world's first artificial nuclear reactor, the Chicago Pile-1, was demonstrated in 1942 at the Metallurgical Laboratory in the University of Chicago, the project designed the X-10 Graphite Reactor and the production reactors at the Hanford Site, in which uranium was irradiated and transmuted into plutonium. In parallel with the work on uranium was an effort to produce plutonium. Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal. The Thin Man gun-type design proved impractical to use with plutonium, so a simpler gun-type design called Little Boy was developed that used uranium-235. The project resulted in two types of atomic bombs, developed concurrently during the war: a relatively simple gun-type fission weapon and a more complex implosion-type nuclear weapon. Research and production took place at more than 30 sites across the US, the UK, and Canada. The Manhattan Project employed nearly 130,000 people at its peak and cost nearly US$2 billion (equivalent to about $26 billion in 2022), over 80 percent of which was for building and operating the plants that produced the fissile material. The project absorbed its earlier British counterpart, Tube Alloys. The Army program was designated the Manhattan District, as its first headquarters were in Manhattan the name gradually superseded the official codename, Development of Substitute Materials, for the entire project. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the bombs. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. It was led by the United States in collaboration with the United Kingdom and with support from Canada. The Manhattan Project was a program of research and development undertaken during World War II to produce the first nuclear weapons.