<Tr> <Th> Shoulder patch that was adopted in 1945 for the Manhattan District </Th> <Td> </Td> </Tr> <Tr> <Th> Manhattan Project emblem (unofficial) </Th> <Td> </Td> </Tr> <P> 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 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 The Army component of the project was designated the Manhattan District; "Manhattan" gradually superseded the official codename, Development of Substitute Materials, for the entire project .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 (about $27 billion in 2016 dollars). 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 . </P> <P> 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 . </P>

Who was the primary scientist that led the manhattan project