Two types of atomic bombs were 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 thirty sites across the United States, the United Kingdom, and Canada. Over 90 percent of the cost was for building factories and to produce fissile material, with less than 10 percent for development and production of the weapons. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (equivalent to about $23 billion in 2020). Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Army component of the project was designated the Manhattan District as its first headquarters were in Manhattan the placename gradually superseded the official codename, Development of Substitute Materials, for the entire project. Nuclear physicist Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the actual 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 with the support of the United Kingdom and Canada. The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons.
