Manhattan Project, the wartime effort to design and build the first nuclear weapons (atomic bombs). With the discovery of fission in 1939, it became clear to scientists that certain radioactive materials could be used to make a bomb of unprecented power. U.S. President Franklin Delano Roosevelt responded by creating the Uranium Committee to investigate this possibility. Progress was slow until Aug., 1942, when the project was placed under U.S. Army control and reorganized. The Manhattan Engineer District (MED) was the official name of the project. The MED's commanding officer, Gen. Leslie R. Groves, was given almost unlimited powers to call upon the military, industrial, and scientific resources of the nation.

A $2-billion effort was required to obtain sufficient amounts of the two necessary isotopes, uranium-235 and plutonium-239. At Oak Ridge, Tenn., the desired uranium-235 was separated from the much more abundant uranium-238 by a laborious process called gaseous diffusion. At the Hanford installation (Wash.), huge nuclear reactors were built to transmute nonfissionable uranium-238 into plutonium-239. This method was based on the principle of the self-sustaining nuclear reaction (nuclear pile) that had first been achieved under the leadership of Enrico Fermi at the metallurgical laboratory of the Univ. of Chicago. At the radiation laboratory of the Univ. of California at Berkeley costly efforts were made to separate the two uranium isotopes using cyclotrons, but only about a gram of pure uranium-235 was obtained. The actual design and building of the plutonium and uranium bombs took place at Los Alamos, N.Mex., under the leadership of J. Robert Oppenheimer. Gathered at this desert laboratory was an extraordinary group of American and European-refugee scientists.

The only nuclear test explosion, code-named Trinity, was of a plutonium device; it took place on July 16, 1945, near Alamogordo, N.Mex. The first uranium bomb ("Little Boy") was delivered untested to the army and was dropped on Hiroshima on Aug. 6, 1945, killing at least 70,000 inhabitants. On Aug. 9, 1945, a plutonium bomb virtually identical to the Trinity device was dropped on Nagasaki, killing at least 35,000 inhabitants.

(1942–45) U.S. government research project that produced the first atomic bomb. In 1939 U.S. scientists urged Pres. Franklin D. Roosevelt to establish a program to study the potential military use of fission, and $6,000 was appropriated. By 1942 the project was code-named Manhattan, after the site of Columbia University, where much of the early research was done. Research also was carried out at the University of California and the University of Chicago. In 1943 a laboratory to construct the bomb was established at Los Alamos, N.M., and staffed by scientists headed by J. Robert Oppenheimer. Production also was carried out at Oak Ridge, Tenn., and Hanford, Wash. The first bomb was exploded in a test at Alamogordo air base in southern New Mexico. By its end the project had cost some $2 billion and had involved 125,000 people.

Learn more about Manhattan Project with a free trial on Britannica.com.