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 The Army component of the project was designated the Manhattan District ; `` Manhattan '' gradually superseded the official codename , Development of Substitute Materials , for the entire project .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 $22 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 .

Question: what was the code name of the us program to develop an atomic bomb
Answer: The Manhattan Project