The Tommy Gun, or Thompson submachine gun, was the creation of General John Thompson, an army guy who developed a few other guns along the way, though none as powerful or infamous as the Tommy Gun.
With WWI still in the distance, Thompson imagined a gun that would be lightweight and capable of laying waste in a spree of bullets. According to the PD, he called it a "trench broom," meaning one could unleash fire across a swath of ground in one grand, violent motion. In 1916, Thompson formed Auto-Ordnance Co. to make the weapon he had dreamed up.
He then sent engineer Theodore Eickhoff, who had previously worked with Thompson on other weapons, to Cleveland because "the Warner & Swasey Co., a manufacturer of instruments and machine tools, would produce parts for the weapon." In a small shop on Euclid, Eickhoff and company began to design and assemble a gun that for all its aims and accomplishments on the battlefield, became best known for its association with gangsters.
That's partly because by the time they were finished, WWI was just about kaput, and the only folks with large bankrolls and a need for a gun that could shoot 1,000 bullets in a minute were Prohibition-era criminals, like infamous "Baby Face" Nelson and John Dillinger.
There's plenty more fascinating history, including the Tommy Guns' adoption and pervasive success in WWII, Eickhoff's regrets about it becoming a trademarked tool of bad guys, so do click over and enjoy.