The atomic bomb was invented during the Manhattan Project, a secret research program that was conducted by the United States during World War II. The development of the atomic bomb was led by a team of scientists, including Robert Oppenheimer, Enrico Fermi, and Leslie Groves, among others.

The idea of creating a nuclear weapon was first proposed by Albert Einstein in a letter to President Franklin D. Roosevelt in 1939. Einstein warned Roosevelt that Nazi Germany might be developing a nuclear weapon and urged the United States to begin its own research into nuclear technology. Roosevelt authorized the creation of the Advisory Committee on Uranium in 1939, which later evolved into the Manhattan Project.

The Manhattan Project was a massive undertaking that involved more than 130,000 people, including scientists, engineers, and support staff. The project was shrouded in secrecy, and the scientists and workers involved were not even told what they were working on until later in the project.

The actual design and construction of the atomic bomb was led by Robert Oppenheimer, a theoretical physicist who had previously worked on the development of nuclear reactors. Oppenheimer oversaw the work of several teams of scientists, who were tasked with designing the bomb and building the necessary components.

The first successful test of the atomic bomb was conducted on July 16, 1945, in Alamogordo, New Mexico. The bomb, which was nicknamed "Trinity," produced an explosion equivalent to 20,000 tons of TNT and demonstrated the destructive power of nuclear weapons.

The atomic bomb was then used in combat for the first and only time during World War II. On August 6, 1945, the United States dropped an atomic bomb on the Japanese city of Hiroshima, killing an estimated 70,000 people instantly and causing long-term health effects for many more. Three days later, the United States dropped another atomic bomb on Nagasaki, killing an estimated 40,000 people.