World War II: The Manhattan Project

Fat Man detonates over Nagasaki, Japan, August 9, 1945
Fat Man detonates over Nagasaki, Japan, August 9, 1945. (US Air Force)

The Manhattan Project was the Allied effort to develop the atomic bomb during World War II. Led by Maj. Gen. Leslie Groves and J. Robert Oppenheimer, it developed research facilities across the United States. The Project was successful and made the atomic bombs used at Hiroshima and Nagasaki.


On August 2, 1939, President Franklin Roosevelt received the Einstein–Szilárd Letter, in which the famed scientists encouraged the United States to develop nuclear weapons lest Nazi Germany create them first. Spurred by this and other committee reports, Roosevelt authorized the National Defense Research Committee to explore nuclear research, and on June 28, 1941, signed Executive Order 8807 which created the Office of Scientific Research & Development with Vannevar Bush as its director. To directly address the need for nuclear research, the NDRC formed the S-1 Uranium Committee under the guidance of Lyman Briggs.

That summer, the S-1 Committee was visited by Australian physicist Marcus Oliphant, a member of the MAUD Committee. The British counterpart of S-1, the MAUD Committee was driving forward in an attempt to create an atomic bomb. As Britain was deeply involved in World War II, Oliphant sought to increase the speed of American research on nuclear matters. Responding, Roosevelt formed a Top Policy Group, consisting of himself, Vice President Henry Wallace, James Conant, Secretary of War Henry Stimson, and General George C. Marshall that October.

Becoming the Manhattan Project

The S-1 Committee held its first formal meeting on December 18, 1941, only days after the attack on Pearl Harbor. Pulling together many of the nation's best scientists including Arthur Compton, Eger Murphree, Harold Urey, and Ernest Lawrence, the group decided to push forward exploring several techniques for extracting uranium-235 as well as different reactor designs. This work progressed at facilities across the country from Columbia University to University of California-Berkeley. Presenting their proposal to Bush and the Top Policy Group, it was approved and Roosevelt authorized funding in June 1942.

As the committee's research would require several large new facilities, it worked in conjunction with the US Army Corps of Engineers. Initially dubbed "Development of Substitute Materials" by the Corps of Engineers, the project was latter re-designated the "Manhattan District" on August 13. During the summer of 1942, the project was led by Colonel James Marshall. Through the summer, Marshall explored sites for facilities but was unable to secure the needed priority from the US Army. Frustrated by a lack of progress, Bush had Marshall replaced in September by newly-promoted Brigadier General Leslie Groves.

The Project Moves Forward

Taking charge, Groves oversaw the acquisition of sites at Oak Ridge, TN, Argonne, IL, Hanford, WA, and, at the suggestion of one of the project's leaders, Robert Oppenheimer, Los Alamos, NM. While work progressed on most of these sites, the facility at Argonne was delayed. As a result, a team working under Enrico Fermi constructed the first successful nuclear reactor at the University of Chicago's Stagg Field. On December 2, 1942, Fermi was able to create the first sustained artificial nuclear chain reaction.

Drawing on resources from across the US and Canada, the facilities at Oak Ridge and Hanford focused on uranium enrichment and plutonium production. For the former, several methods were used including electromagnetic separation, gaseous diffusion, and thermal diffusion. As research and production moved forward under a cloak of secrecy, research on nuclear matters was shared with the British. Signing the Quebec Agreement in August 1943, the two nations agreed to collaborate on atomic matters. This led to several notable scientists including Niels Bohr, Otto Frisch, Klaus Fuchs, and Rudolf Peierls joining the project.

Weapon Design

As production ensued elsewhere, Oppenheimer and the team at Los Alamos worked on designing the atomic bomb. Early work focused "gun-type" designs which fired one piece of uranium into another to create a nuclear chain reaction. While this approach proved promising for uranium-based bombs, it was less so for those utilizing plutonium. As a result, the scientists at Los Alamos began developing an implosion design for a plutonium-based bomb as this material was relatively more plentiful. By July 1944, the bulk of the research was focused on the plutonium designs and the uranium gun-type bomb was less of a priority.

The Trinity Test

As the implosion-type device was more complex, Oppenheimer felt that a test of the weapon was needed before it could be moved into production. Though plutonium was relatively scarce at the time, Groves authorized the test and assigned planning for it to Kenneth Bainbridge in March 1944. Bainbridge pushed forward and selected the Alamogordo Bombing Range as the detonation site. Though he originally planned to use a containment vessel to recover the fissile material, Oppenheimer later elected to abandon it as plutonium had become more available.

Dubbed the Trinity Test, a pre-test explosion was conducted on May 7, 1945. This was followed by the construction of a 100-ft. tower at the site. The implosion test device, nicknamed "The Gadget," was hoisted to the top to simulate a bomb falling from an aircraft. At 5:30 AM on July 16, with all the key Manhattan Project members present, the device was successfully detonated with the energy equivalent of around 20 kilotons of TNT. Alerting President Harry S. Truman, then at the Potsdam Conference, the team began moving to build atomic bombs using the test's results.

Little Boy & Fat Man

Though the implosion device was preferred, the first weapon to leave Los Alamos was a gun-type design, as the design was thought more reliable. Components were carried to Tinian aboard the heavy cruiser USS Indianapolis and arrived on July 26. With Japan's refusal of calls to surrender, Truman authorized the bomb's use against the city of Hiroshima. On August 6, Colonel Paul Tibbets departed Tinian with the bomb, dubbed "Little Boy," aboard the B-29 Superfortress Enola Gay.

Released over the city at 8:15 AM, Little Boy fell for fifty-seven seconds, before detonating at the predetermined height of 1,900 feet with a blast equivalent to about 13-15 kilotons of TNT. Creating an area of complete devastation approximately two miles in diameter, the bomb, with its resulting shock wave and fire storm, effectively destroyed around 4.7 square miles of the city, killing 70,000-80,000 and injuring another 70,000. Its use was quickly followed three days later when "Fat Man," an implosion plutonium bomb, fell on Nagasaki. Generating a blast equivalent of 21 kilotons of TNT, it killed 35,000 and wounded 60,000. With the use of the two bombs, Japan quickly sued for peace.


Costing nearly $2 billion and employing approximately 130,000 people, the Manhattan Project was one of the US' largest endeavors during World War II. Its success ushered in the nuclear age, which saw nuclear power harnessed for both military and peaceful purposes. Work on nuclear weapons continued under the Manhattan Project's jurisdiction and saw further testing in 1946 at Bikini Atoll. Control of nuclear research passed to the United States Atomic Energy Commission on January 1, 1947, following the passage of the Atomic Energy Act of 1946. Though a highly secret program, the Manhattan Project was penetrated by Soviet spies, including Fuchs, during the war. As a result of his work, and that of others such as Julius and Ethel Rosenberg, the US' atomic hegemony ended in 1949 when the Soviets detonated their first nuclear weapon.

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Hickman, Kennedy. "World War II: The Manhattan Project." ThoughtCo, Aug. 26, 2020, Hickman, Kennedy. (2020, August 26). World War II: The Manhattan Project. Retrieved from Hickman, Kennedy. "World War II: The Manhattan Project." ThoughtCo. (accessed May 17, 2021).