DC's first superhero called the Atom was a guy named Al Pratt. During the Golden Age of comics (in other words, around World War II), Al was a short kid (just 5' 1" tall) who got picked on and beat up a lot. He got tired of being the target of bullies and got some training from a boxing pro. He improved his physique and became an expert fighter; he had a shot at the professional boxing title, but chose to wear a funny costume and fight crime instead. Though he had no superpowers, he was quite successful and became one of the founding members of the Justice Society of America--of course, just about everyone else had powers, so he was just the short guy who was a good fighter, which wasn't that great for his confidence level. Later in his superheroic career, Al fought an atomic-powered enemy and, in the process, gained an "Atomic Punch"--a power that allowed him to hit things really, really hard.

Al retired from full-time superhero duties in the 1950s, but returned to the long-underwear game from time to time. He got married, fathered a son who was secretly kidnapped and eventually became a superhero called Damage (though Al and his son never met), and became the godfather for a boy who would eventually become a superhero called Nuklon. Al died during the "Zero Hour" crisis when a time-controlling villain called Extant aged a group of JSAers unnaturally.

The Atom was created by Bill O'Conner and Ben Flinton and made his first appearance in "All-American Comics" #19 in October of 1940.

Many thanks to Habakkuk, who provided additional sources for research. :)