page_title: Atomic bombings of Hiroshima and Nagasaki
section_title: Atomic bomb development
hierarchical_section_title: Atomic bombings of Hiroshima and Nagasaki / Background / Atomic bomb development
caption_attribution_description: General Groves at Oak Ridge For more information or additional images, please contact 202-586-5251.
context_page_description: The United States detonated two nuclear weapons over the Japanese cities of Hiroshima and Nagasaki on August 6 and 9, 1945, respectively. The two bombings killed between 129,000 and 226,000 people, most of whom were civilians, and remain the only uses of nuclear weapons in armed conflict.
  In the final year of World War II, the Allies prepared for a costly invasion of the Japanese mainland. This undertaking was preceded by a conventional and firebombing campaign that devastated 67 Japanese cities. The war in Europe concluded when Germany surrendered on May 8, 1945, and the Allies turned their full attention to the Pacific War. By July 1945, the Allies' Manhattan Project had produced two types of atomic bombs: "Fat Man", a plutonium implosion-type nuclear weapon; and "Little Boy", an enriched uranium gun-type fission weapon. The 509th Composite Group of the United States Army Air Forces was trained and equipped with the specialized Silverplate version of the Boeing B-29 Superfortress, and deployed to Tinian in the Mariana Islands.
context_section_description: The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, made the development of an atomic bomb a theoretical possibility. Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein-Szilard letter. This prompted preliminary research in the United States in late 1939. Progress was slow until the arrival of the British MAUD Committee report in late 1941, which indicated that only 5 to 10 kilograms of isotopically enriched uranium-235 were needed for a bomb instead of tons of natural uranium and a neutron moderator like heavy water.
  The 1943 Quebec Agreement merged the nuclear weapons projects of the United Kingdom and Canada, Tube Alloys and the Montreal Laboratory, with the Manhattan Project, under the direction of Major General Leslie R. Groves, Jr., of the U.S. Army Corps of Engineers. Groves appointed J. Robert Oppenheimer to organize and head the project's Los Alamos Laboratory in New Mexico, where bomb design work was carried out. Two types of bombs were eventually developed, both named by Robert Serber. Little Boy was a gun-type fission weapon that used uranium-235, a rare isotope of uranium separated at the Clinton Engineer Works at Oak Ridge, Tennessee. The other, known as a Fat Man device, was a more powerful and efficient, but more complicated, implosion-type nuclear weapon that used plutonium created in nuclear reactors at Hanford, Washington.
  There was a Japanese nuclear weapon program, but it lacked the human, mineral and financial resources of the Manhattan Project, and never made much progress towards developing an atomic bomb.
caption: Leslie Groves, Manhattan Project director, with a map of Japan
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