This is a good and tough question. There is no who in the process of building the atomic bomb, because it was the result of several years of research and discoveries in the field of Physics. There are several opinions of who was the most important person or the inventor of the atomic bomb, but I would rather think of all the physicists that contributed to make the atomic bomb, instead of looking for only one or two inventors.

Going back on time, we can say that it was the Italian scientist Enrico Fermi who first contributed to the atomic bomb. He worked on bombarding artificial radioactivity with neutrons in order to get very heavy elements, even heavier than uranium. By 1939, Leo Szilard, a Hungarian scientist working at Columbia University tried to confirm the reality of atomic fission, the basic idea of the atomic bomb. Due to his work, some people thing that he is the inventor of the atomic bomb. The physicist Albert Einstein did not directly participate in the invention of the atomic bomb, but he was instrumental in facilitating its development.

Finally by 1942, J. Robert Oppenheimer was made director of Los Alamos lab, and he gathered about 200 of the best scientists in the field to live and work there. They designed two bombs, one using uranium (called "Little Boy") and one using plutonium ("Fat Man"). On July 16, 1945 the atomic bomb was detonated, producing an intense flash of light seen by observers in bunkers 10 km away and a fireball that expanded to 600 meters in two seconds. It grew to a height of more than 12 kilometers, boiling up in the shape of a mushroom. Forty seconds later, the blast of air from the bomb reached the observation bunkers, along with a long and deafening roar of sound. The explosive power, equivalent to 18.6 kilotons of TNT, was almost four times larger than predicted.

Hundreds of Physics scientists invented and built the atomic bomb.