Timeline of the Manhattan Project

The Manhattan Project was a research and development project that produced the first atomic bombs during World War II. Led by the United States with support from the United Kingdom and Canada, it was directed by Major General Leslie Groves of the US Army Corps of Engineers. Starting modestly in 1939, the project grew to involve more than 130,000 people and cost nearly US$2 billion, which is about $36.3 billion today.

Two main types of atomic bombs were developed. One used uranium-235, a rare isotope that was difficult to separate from more common uranium-238. Three methods were used for this separation: electromagnetic, gaseous, and thermal, mostly at Oak Ridge, Tennessee. The other bomb used plutonium, produced in reactors at Oak Ridge and Hanford, Washington. Because plutonium couldn’t use the simple gun-type design, a more complex implosion-type nuclear weapon was created at the main laboratory in Los Alamos, New Mexico.

This timeline covers important events from before the official start of the Manhattan Project through the atomic bombings of Hiroshima and Nagasaki, until the project was replaced by the Atomic Energy Commission in 1947. It shows how scientific discoveries and engineering efforts came together to change history forever.

1939

In August 1939, physicist Leó Szilárd wrote a letter signed by Albert Einstein and sent to President Franklin D. Roosevelt. The letter warned that Nazi Germany might develop a powerful weapon using nuclear fission, so Roosevelt agreed to fund research on the idea.

Soon after, on September 3, Great Britain and France declared war on Nazi Germany after its invasion of Poland, marking the start of World War II. Later that month, economist Alexander Sachs gave the letter to President Roosevelt, who created a special group called the Advisory Committee on Uranium. The group’s first meeting happened on October 21, and they set aside money for early experiments with neutron science.

1940

In 1940, important early steps were taken toward understanding atomic energy. In March, scientists at Columbia University confirmed that uranium 235 could cause atomic fission. Around the same time, scientists in Britain wrote a report showing that a very small amount of enriched uranium could create an atomic bomb.

Later that year, new groups were formed to study this idea further, and funding was provided for uranium research. Additionally, a large amount of uranium was secretly moved to the United States from a mine in Africa.

1941

In 1941, important steps were taken toward developing nuclear weapons. On February 25, scientists Glenn Seaborg and Arthur Wahl at the University of California, Berkeley discovered plutonium. Later, in May, a report by Arthur Compton and the National Academy of Sciences showed that creating nuclear power for military use looked possible.

After Japan attacked Pearl Harbor on December 7, 1941, the United States entered World War II, which increased the urgency of the nuclear weapons project. The Office of Scientific Research and Development was created to manage this important research.

1942

In January 1942, President Roosevelt officially approved the atomic bomb project. Throughout the year, important decisions were made to organize and advance the work. Scientists like Compton chose the University of Chicago to focus on plutonium research, and a group of leaders formed the S-1 Executive Committee to guide the project.

Key events included the selection of Stone & Webster to build facilities in Tennessee, and Robert Oppenheimer organizing discussions on bomb design at the University of California, Berkeley. General Leslie Groves took charge of the project in September, and uranium ore was secured from Congo for processing. By December, the first nuclear reactor, Chicago Pile-1, achieved a self-sustaining reaction, marking a major scientific breakthrough.

1943

In 1943, major steps were taken to build the first atomic bombs. In January, General Groves approved the development of the Hanford Site. Construction began on large facilities for separating uranium at Oak Ridge and for enriching uranium through gaseous diffusion. The Los Alamos laboratory was established in April to bring scientists together to design the bombs.

Important meetings and agreements were made throughout the year. In July, Los Alamos, Oak Ridge, and Hanford were declared military districts. In August, the first sample of plutonium arrived at Los Alamos, and the Medical Section of the Manhattan Project was created. In September, the Quebec Agreement was signed, merging the British Tube Alloys project with the Manhattan Project and forming the Combined Policy Committee to coordinate efforts among the United States, United Kingdom, and Canada. Construction also began on the first reactor at the Hanford Site, and the X-10 Graphite Reactor at Oak Ridge became operational in November.

1944

In 1944, important developments happened in the Manhattan Project. On January 11, a special group led by Edward Teller began studying a new way to create explosions at Los Alamos. By April 5, scientist Emilio Segrè found that a type of material from Oak Ridge wasn’t suitable for one kind of bomb design.

Later in the year, major changes were made to focus on a new bomb design called "Fat Man". On September 26, a large nuclear reactor began operating at the Hanford Site. By December, a special group was formed to prepare for using the new bombs, showing how close scientists were to creating history.

1945

In January, Brigadier General Thomas Farrell became a deputy to Groves. Early tests and preparations continued, including the first test explosion of a plutonium device on July 16 at Alamogordo, known as the Trinity nuclear test.

Key events led up to the use of atomic bombs on Japan. On August 6, the B-29 Enola Gay dropped a bomb named Little Boy on Hiroshima. On August 9, another B-29 named Bockscar dropped a bomb named Fat Man on Nagasaki. These events led to Japan’s surrender on August 14, ending World War II.

1946

In 1946, important events shaped the future of atomic energy. In February, news about a Russian spy ring in Canada caused concern in the United States about how to control atomic research after the war. Later that year, on July 1 and July 25, two tests called Able and Baker were conducted at Bikini Atoll as part of Operation Crossroads. Finally, on August 1, President Truman signed the Atomic Energy Act of 1946, which established new rules for managing atomic research in the United States.

1947

In 1947, important changes happened to the Manhattan Project. On January 1, the Atomic Energy Act of 1946, also called the McMahon Act, began. This act moved the Manhattan Project to the United States Atomic Energy Commission. Later, on August 15, the Manhattan District, which was part of the project, was closed down.