Atom

The Mighty Mite
Al Pratt was born in 1920. Pratt was a sophomore at Calvin College in 1940 and suffered indignity due to his short stature. This led to him becoming the target of harassment and bullying, and referred to as "Atom Al" by his tormentors. Due to this, he felt he was unable to impress Mary James, the girl of his dreams. One day, feeling sympathetic, he bought dinner for a vagrant who turned out to be Joe Morgan, a former boxing champ. Morgan promised Pratt the young man could be made into a muscular man with the right training. In under one year Pratt became a massively strong fighter with unbelievable strength, and he decided to use his new ability to protect the oppressed as the “mystery man,” Atom.

World War II
At the start of World War II, President Roosevelt organized the "mystery men" to band together and create the Justice Society of America. Al was a founding member and later joined the All-Star Squadron where he struck up a strong friendship with fellow member Wildcat. He was present when the JSA battled Ian Karkull. During the fight, Atom was bathed in mystical energy that kept him young for decades.

The Human Atomic Bomb
In another battle alongside the All-Star Squadron, Al Pratt fought the reluctant villain known as Cyclotron. Energy released in that battle caused Al Pratt to develop radiation immunity by the late-1940s. But when the Atom and Starman went to the Manhattan Project site to stop a foe, Pratt, without thinking, ran through the fallout to save Starman and was caught in another explosion. He was unaffected by this and their foe was defeated. Surviving the blast completely unharmed, it was discovered that the extra dose of radiation gave him more superpowers.