<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The Manhattan Project was a research and development project that produced the first atomic bombs during World War II . 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 US Army Corps of Engineers . The Army component of the project was designated the Manhattan District; "Manhattan" gradually became the codename 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 (about $27 billion in 2017 dollars). Over 90% of the cost was for building factories and producing the fissionable materials, with less than 10% for development and production of the weapons . </P> <P> Two types of atomic bomb were developed during the war . A relatively simple gun - type fission weapon was made using uranium - 235, an isotope that makes up only 0.7 percent of natural uranium . Since it is chemically identical to the most common isotope, uranium - 238, and has almost the same mass, it proved difficult to separate . Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal . Most of this work was performed at Oak Ridge, Tennessee . In parallel with the work on uranium was an effort to produce plutonium . Reactors were constructed at Oak Ridge and Hanford, Washington, in which uranium was irradiated and transmuted into plutonium . The plutonium was then chemically separated from the uranium . The gun - type design proved impractical to use with plutonium so a more complex implosion - type nuclear weapon was developed in a concerted design and construction effort at the project's principal research and design laboratory in Los Alamos, New Mexico . </P>

When did the manhattan project began and end