The Manhattan Project was the effort, led by the United States with participation from the United Kingdom and Canada, which resulted in the development of the firstatomic bomb during World War II. From 1942 to 1946 the project was under the command of Major General Leslie R. Groves Jr. of the US Army Corps of Engineers. The Army component of the project was designated the Manhattan District orManhattan Engineer District (MED), but "Manhattan" gradually superseded the official codename for the project.
The project had its roots in the Einstein–Szilárd letter, which warned that Nazi Germany might develop nuclear weapons. The letter was written by prominent physicists, signed by Albert Einstein, and delivered to President Franklin D. Roosevelt in October 1939. The Manhattan Project, which began as a small research program that year, eventually employed more than 130,000 people at a
Manhattan Engineer Shoulder Patch
cost of nearlyUS$2 billion. Research and production took place at more than 30 sites, some secret, including universities across the United States, the United Kingdom and Canada. The three primary research and production sites of the project were theplutonium-production facility at the Hanford Site in eastern Washington state, theuranium enrichment facilities at Oak Ridge, Tennessee, and the weapons research and design laboratory at Los Alamos, New Mexico.
Two types of atomic bombs were developed during the war. A gun-type fission weapon was made from uranium-235, an isotope of uranium that makes up only 0.7 percent of natural uranium. This isotope proved difficult to separate from the main isotope, uranium-238, since it was chemically identical and almost the same weight. Three methods were employed for isotope separation: electromagnetic, gaseous and thermal. Most of this work was performed at Oak Ridge. This design proved impractical to use with plutonium so an implosion-type nuclear weapon was developed through a concerted design and construction effort at Los Alamos. An implosion bomb was the first nuclear device ever detonated, at the Trinity test on 16 July 1945. A gun-type weapon, Little Boy, was dropped at Hiroshima on 6 August 1945, while a more complex plutonium-core weapon, Fat Man, was dropped atNagasaki three days later.
The Manhattan Project was also charged with gathering intelligence on the German nuclear energy project. Through Operation Alsos, Manhattan Project personnel served in Europe, sometimes behind enemy lines, where they gathered nuclear materials and rounded up German scientists. The MED maintained control over American atomic weapons production until the formation of the United States Atomic Energy Commission in January 1947.