Claim: Leslie Groves was a United States Army Corps of Engineers officer in the 1940s.

The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons . It was led by the United States with the support of the United Kingdom and Canada . From 1942 to 1946 , the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers . Nuclear physicist J. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the actual bombs . The Army component of the project was designated the Manhattan District ; `` Manhattan '' gradually superseded the official codename , Development of Substitute Materials , for the entire project . Along the way , the project absorbed its earlier British counterpart , Tube Alloys . The Manhattan Project began modestly in 1939 , but grew to employ more than 130,000 people and cost nearly US $ 2 billion -LRB- about $ in dollars -RRB- . Over 90 % of the cost was for building factories and to produce fissile material , with less than 10 % for development and production of the weapons . Research and production took place at more than 30 sites across the United States , the United Kingdom , and Canada .   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 . The Thin Man gun-type design proved impractical to use with plutonium so a simpler gun-type called Little Boy was developed that used uranium-235 , an isotope that makes up only 0.7 percent of natural uranium . Chemically identical to the most common isotope , uranium-238 , and with almost the same mass , it proved difficult to separate the two . Three methods were employed for uranium enrichment : electromagnetic , gaseous and thermal . Most of this work was performed at the Clinton Engineer Works at Oak Ridge , Tennessee .   In parallel with the work on uranium was an effort to produce plutonium . After the feasibility of the world 's first artificial nuclear reactor was demonstrated in Chicago at the Metallurgical Laboratory , it designed the X-10 Graphite Reactor at Oak Ridge and the production reactors in Hanford , Washington , in which uranium was irradiated and transmuted into plutonium . The plutonium was then chemically separated from the uranium . The Fat Man implosion-type weapon was developed in a concerted design and development effort by the Los Alamos Laboratory .   The project was also charged with gathering intelligence on the German nuclear weapon project . Through Operation Alsos , Manhattan Project personnel served in Europe , sometimes behind enemy lines , where they gathered nuclear materials and documents , and rounded up German scientists . Despite the Manhattan Project 's tight security , Soviet atomic spies still penetrated the program .   The first nuclear device ever detonated was an implosion-type bomb at the Trinity test , conducted at New Mexico 's Alamogordo Bombing and Gunnery Range on 16 July 1945 . Little Boy and Fat Man bombs were used a month later in the atomic bombings of Hiroshima and Nagasaki , respectively . In the immediate postwar years , the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads , developed new weapons , promoted the development of the network of national laboratories , supported medical research into radiology and laid the foundations for the nuclear navy . It maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission in January 1947 .