Manhattan Project: The Secret Race to Build the First Atomic Bomb

The Manhattan Project was a massive, top-secret research and development program undertaken during World War II. Led by the United States in collaboration with the United Kingdom and Canada, its singular goal was to produce the world's first nuclear weapons. At its peak, the project employed nearly 130,000 people and cost approximately US$2 billion—roughly $28 billion in 2024 currency.

The project transformed theoretical physics into a weapon of unprecedented power, culminating in the development of two distinct bomb designs and the first-ever nuclear detonation. This effort not only altered the course of World War II but fundamentally changed global geopolitics and science.

Oval shaped shoulder patch with a deep blue background. At the top is a red circle and blue star, the patch of the Army Service Forces. It is surrounded by a white oval, representing a mushroom cloud. Below it is a white lightning bolt cracking a yellow circle, representing an atom.

Key Facts

Organization chart of the project, showing project headquarters divisions at the top, Manhattan District in the middle, and field offices at the bottom — The Manhattan Project Organizational Chart, 1 May 1946

Timeline: Active from 1942 to 1946; officially disbanded on August 15, 1947.
Primary Sites: Los Alamos (design), Oak Ridge (uranium), and Hanford (plutonium).
Weapon Types: Little Boy (uranium gun-type) and Fat Man (plutonium implosion).
First Test: The Trinity test on July 16, 1945, in New Mexico.
Total Cost: Nearly US$2 billion (1945 USD).

Origins and International Collaboration

A series of doodles — Different fission bomb assembly methods explored during the July 1942 conference (sketches created in 1943 by Robert Serber)

The path to the atomic bomb began with breakthroughs in nuclear fission (the splitting of an atomic nucleus). In June 1939, Otto Robert Frisch and Rudolf Peierls at the University of Birmingham calculated that the critical mass of uranium-235 was small enough—roughly 10 kilograms—to be carried by aircraft. This finding led to the British MAUD Committee's recommendation to pursue an atomic weapon.

By July 1940, Britain shared its research with the United States. The Tizard Mission, featuring John Cockcroft, revealed that the American effort was then smaller and less advanced than the British one. This sparked a surge in U.S. activity, eventually leading to the formation of the S-1 Committee and the subsequent establishment of the Manhattan District under the U.S. Army Corps of Engineers.

Enrico Fermi, John R. Dunning, and Dana P. Mitchell in front of the cyclotron in the basement of Pupin Hall at Columbia University, 1940

The project's momentum accelerated in 1942 when Vannevar Bush forwarded a report on "Atomic Fission Bombs" to President Franklin Roosevelt. Roosevelt approved the "crash program" with a simple handwritten note: "V.B. OK. FDR."

Six men in suits sitting on chairs, smiling and laughing — March 1940 meeting at Berkeley, California: Ernest O. Lawrence, Arthur H. Compton, Vannevar Bush, James B. Conant, Karl T. Compton, and Alfred L. Loomis

Cover letter written by Vannevar Bush, forwarding the report on "Atomic Fission Bombs" to President Franklin Roosevelt in 1942, recommending the approval of a crash program to build an atomic bomb. Roosevelt approved it by writing on the front of it: "V.B. OK. FDR."

The Industrial Engine: Production Sites

A man smiling in a suit in suit and one in a uniform chat around a pile of twisted metal. — Oppenheimer and Groves at the remains of the Trinity test in September 1945, two months after the test blast and just after the end of World War II. The white overshoes prevented fallout from sticking to the soles of their shoes.[52]

Creating a nuclear weapon required massive industrial infrastructure to produce fissile materials. The project was divided into several specialized hubs:

Oak Ridge, Tennessee

Oak Ridge focused on uranium enrichment. Natural uranium consists of 99.3% uranium-238 and only 0.7% uranium-235; only the latter is fissile. To separate them, the project utilized three successful technologies: electromagnetic separation (Y-12), gaseous diffusion (K-25), and thermal diffusion (S-50). By March 1945, these processes were run in series to boost uranium-235 levels to approximately 89%.

Contour map of the Oak Ridge area. There is a river to the south, while the township is in the north. — Oak Ridge hosted several uranium separation technologies. The Y-12 electromagnetic separation plant is in the upper right. The K-25 and K-27 gaseous diffusion plants are in the lower left, near the S-50 thermal diffusion plant. The X-10 was for plutonium production.

Oblique aerial view of an enormous U-shaped building — Oak Ridge K-25 plant

Hanford, Washington

While Oak Ridge handled uranium, Hanford was dedicated to plutonium production. Using the B-Reactor, scientists created plutonium, a synthetic element. This process was highly dangerous; by the end of the war, half of the chemists and metallurgists had to be removed from work due to high levels of plutonium detected in their urine.

An aerial view of the Hanford B-Reactor site from June 1944. At center is the reactor building. Small trucks dot the landscape and give a sense of scale. Two large water towers loom above the plant. — Aerial view of Hanford B-Reactor site, June 1944

Los Alamos, New Mexico (Project Y)

Los Alamos served as the primary design laboratory. Here, scientists worked on the complex physics of the bomb. While the uranium gun-type design was relatively straightforward, the plutonium bomb required a complex implosion method using explosive lenses because plutonium-240 contamination made it unsuitable for a gun-type device.

Map of Los Alamos site, New Mexico, 1943–1945

The Path to Detonation

A large man in uniform and a bespectacled thin man in a suit and tie sit at a desk. — Groves confers with James Chadwick, the head of the British Mission.

The culmination of these efforts was "The Gadget," an implosion-type device. On July 16, 1945, the Trinity test at the White Sands Proving Ground became the first nuclear explosion in history.

The Trinity test of the Manhattan Project was the first detonation of a nuclear weapon.

Following the successful test, the United States deployed two bombs on Japan in August 1945: Little Boy (enriched uranium) on Hiroshima on August 6, and Fat Man (plutonium) on Nagasaki on August 9. These remain the only uses of nuclear weapons in conflict.

Two mushroom clouds rise vertically. — The mushroom clouds of the atomic attacks on Hiroshima, Japan, 6 August 1945 (left) and Nagasaki, Japan, 9 August 1945 (right)

Project Costs and Logistics

Workers, mostly women, pour out of a cluster of buildings. A billboard exhorts them to "Make C.E.W. COUNT continue to protect project information!" — Shift change at the Y-12 uranium enrichment facility at the Clinton Engineer Works in Oak Ridge, Tennessee, on 11 August 1945. By May 1945, 82,000 people were employed at the Clinton Engineer Works.[82] Photograph by the Manhattan District photographer Ed Westcott.

The scale of the Manhattan Project was unprecedented, requiring a vast network of contractors and strict security measures to prevent espionage from foreign intelligence, including Soviet spies.

Manhattan Project Expenditure by Site (1945 USD)
Site/Category	Cost (Millions USD)	2024 Adjusted Cost (Millions)	% of Total
Oak Ridge	$1,188	$16,368	62.9%
Hanford	$390	$5,374	20.6%
Special Operating Materials	$103	$1,424	5.5%
Los Alamos	$74	$1,020	3.9%
Research and Development	$70	$960	3.7%
Government Overhead	$37	$513	2.0%
Heavy Water Plants	$27	$369	1.4%
Total	$1,890	$26,027	100%

Frequently Asked Questions

Some of the University of Chicago team that worked on the Chicago Pile-1, the first nuclear reactor, including Enrico Fermi and Walter Zinn in the front row and Harold Agnew, Leona Woods and Leó Szilárd in the second

A large crowd of sullen looking workmen at a counter where two women are writing. Some of the workmen are wearing identify photographs of themselves on their hats. — Hanford workers collect their paychecks at the Western Union office.

A sample of a high-quality uranium-bearing ore (Tobernite) from the Shinkolobwe mine in Belgian Congo

A uranium metal "biscuit" created from the reduction reaction of the Ames process

A large oval-shaped structure — Alpha I racetrack at Y-12

A long corridor with many consoles with dials and switches, attended by women seated on high stools — The Calutron Girls were young women who monitored calutron control panels at Y-12. Gladys Owens, seated in the foreground, was unaware of what she had been involved in.[160]

A factory with three smoking chimneys on a river bend, viewed from above — The S-50 plant is the dark building to the upper left behind the Oak Ridge powerhouse (with smokestacks).

Two workmen on a movable platform similar to that used by window washers, stick a rod into one of many small holes in the wall in front of them. — Workers load uranium slugs into the X-10 Graphite Reactor.

A contour map showing the fork of the Columbia and Yakima rivers and the boundary of the land, with seven small red squares marked on it — Map of the Hanford Site. Railroads flank the plants to the north and south. Reactors are the three northernmost red squares, along the Columbia River. The separation plants are the lower two red squares from the grouping south of the reactors. The bottom red square is the 300 area.

Long, tube-like casings. In the background are several ovoid casings and a tow truck. — A row of Thin Man casings. Fat Man casings are visible in the background.

Diagram showing fast explosive, slow explosive, uranium tamper, plutonium core and neutron initiator — An implosion-type nuclear bomb

A shack surrounded by pine trees. There is snow on the ground. A man and a woman in white lab coats are pulling on a rope, which is attached to a small trolley on a wooden platform. On top of the trolley is a large cylindrical object. — Remote handling of a kilocurie source of radiolanthanum for a RaLa Experiment at Los Alamos

Men stand around a large oil-rig type structure. A large round object is being hoisted up. — The explosives of "the gadget" were raised to the top of the tower for the final assembly.

Manhattan Project contractors' employment, August 1942-December 1946

A large crowd of men and women in uniform listens to a fat man in uniform speaking at a microphone. They are wearing the Army Service Forces sleeve patch. The women are at the front and the men at the back. Beside him is the flag of the Army Corps of Engineers. Behind them are wooden two-storey buildings. — Major General Leslie R. Groves Jr., speaks to service personnel Oak Ridge Tennessee in August 1945.

Uncle Sam has removed his hat and is rolling up his sleeves. On the wall in front of him are three monkeys and the slogan: What you see here/ What you do here/ What you hear here/ When you leave here/ Let it stay here. — A billboard encouraging secrecy among Oak Ridge workers

Security poster, warning office workers to close drawers and put documents in safes when not being used

Soldiers and workmen, some wearing steel helmet, clamber over what looks like a giant manhole. — Allied soldiers dismantle the German experimental nuclear reactor at Haigerloch.

A shiny metal four-engined aircraft stands on a runway. The crew pose in front of it. — Silverplate B-29 Straight Flush. The tail code of the 444th Bombardment Group is painted on for security reasons.

Men in suits and uniforms stand on a dais decorated with bunting and salute. — Presentation of the Army–Navy "E" Award at Los Alamos on 16 October 1945. Standing, left to right: J. Robert Oppenheimer, unidentified, unidentified, Kenneth Nichols, Leslie Groves, Robert Gordon Sproul, William Sterling Parsons.

A man in a suit is seated at a desk, signing a document. Seven men in suits gather around him. — President Harry S. Truman signs the Atomic Energy Act of 1946, establishing the United States Atomic Energy Commission.

Manhattan Project monthly expenditures from January 1943 through the end of December 1946. In its peak month, August 1944, US$111.4 million was spent on the project.

The Lake Ontario Ordnance Works (LOOW) near Niagara Falls became a principal repository for Manhattan Project waste for the Eastern United States.[362] All of the radioactive materials stored at the LOOW site—including thorium, uranium, and the world's largest concentration of radium-226—were buried in an "Interim Waste Containment Structure" (in the foreground) in 1991.[363][364][365]

Why were two different bomb designs created?

The project developed both a uranium gun-type design (Little Boy) and a plutonium implosion design (Fat Man). The implosion design became necessary because plutonium produced at Hanford contained too much plutonium-240, which would cause a premature detonation in a gun-type weapon.

What was the purpose of the Trinity test?

The Trinity test was the first detonation of a nuclear weapon. It was conducted to prove that the complex implosion-type design worked before the weapon was used in combat.

How was uranium enriched for the bombs?

Uranium was enriched at Oak Ridge using a series of processes: thermal diffusion (S-50) first raised the concentration of uranium-235, followed by gaseous diffusion (K-25), and finally electromagnetic separation (Y-12) to reach the necessary 89% purity.

Who led the Manhattan Project?

The project was a joint effort between military and scientific leadership. Key figures included Major General Leslie R. Groves Jr., who oversaw the military and construction aspects, and J. Robert Oppenheimer, who led the scientific research at Los Alamos.

What happened to the project after the war?

The project was gradually wound down, with many facilities like the Y-12 Alpha tracks closing by late 1945. The Manhattan District was officially disbanded on August 15, 1947, and the Atomic Energy Act of 1946 established the U.S. Atomic Energy Commission to manage nuclear materials.