On December 28, 1942, United States president Franklin D. Roosevelt formally approved an investment in the top-secret project that would result in atomic bombs being dropped on the Japanese cities of Hiroshima and Nagasaki three years later, killing thousands of people.

The approval of detailed plans for building production facilities was made after a series of more incremental decisions committing the nation to creating atomic weapons.

Years in the making

This research and development project moved slowly — in the late 1930s, there were anxieties that Nazi Germany might build an atomic bomb first, and the Manhattan Project became the Allied Forces' effort to beat it.

In 1939, Hungarian-American physicist Leo Szilard — who had conceived of the idea of a nuclear chain reaction — helped draft what is called the "Einstein-Szilard letter". 

Physicist Albert Einstein with Robert Oppenheimer in 1950. (Wikipedia)

The letter, signed by Albert Einstein, was sent to Roosevelt and argued that the US should start its own nuclear program and stockpile uranium ore, warning that Germany could develop atomic bombs.

Then, on December 7, 1941, Japan launched a surprise attack on an American fleet stationed at Pearl Harbor, on the Hawaiian island of Oahu, marking a major turning point in WWII as the US was drawn into the conflict.

A secret program

In June 1942, the US Army Corps of Engineers created the Manhattan Engineer District to hide the development of the atomic bomb during the war, and the project's first officers were in a Manhattan skyscraper.

Roosevelt initially allocated $US500 million to the large-scale undertaking that involved thousands of workers, many of whom did not know the goal of the project was to build a new type of bomb.

The weapons development portion of the project was overseen by physicist J. Robert Oppenheimer, who is often referred to as the "father of the atomic bomb".

Last year, the New York Times investigated an overlooked question — how did the Roosevelt administration pay for the $US2 billion top-secret project without Congress noticing? 

There was no debate when Congress voted to fund the bomb; in fact, only seven lawmakers in the entire Congress "had any idea that they were approving $US800 million — the equivalent of $US13.6 billion today — to create a weapon of mass destruction that would soon kill and maim more than 200,000 people".

The bomb is tested

In April 1945, Roosevelt died and was replaced by his vice president, Harry Truman, who was then briefed about the secret plan to build a powerful nuclear weapon.

The detonation of the world's first nuclear weapon, known as the Trinity test and part of the Manhattan Project. Photo taken July 16, 1945. (US Department of Energy: Jack Aeby)

Two months later, the project culminated in a test known as Trinity — the first detonation of a nuclear weapon. 

Although the Manhattan Project was top secret, the explosion would be heard and seen for hundreds of kilometres, so the plutonium bomb, like the "Fat Man" bomb that eventually dropped on Nagasaki, was detonated in the Jornada del Muerto desert in New Mexico. 

The second type of atomic bomb developed in the program was an enriched uranium gun-type fission weapon, like the "Little Boy" dropped on the city of Hiroshima.