The story of the French 75 goes back to around 1915 when Harry MacElhone created in at the New York Bar in Paris. It was brought to the U.S. by returning World War I pilots and became a popular drink at New York City's Stork Club. The name comes from a 75mm French field gun that was said to have the same kick as the drink.
At some point in its early history this drink was made with Cognac in place of the gin and there is some question as to which version is the real French 75, but gin is the more common now. To add another twist, if the same drink is made with vodka for the base spirit, it is a French 76.