From the Dairy Queen Company Website.
On August 4, 1938, a father and son, John Fremont McCullough and son Bradley, were in a mix plant in Green River and had been experimenting with a soft frozen dairy product and contacted friend and customer Sherb Noble. He agreed to run an "all you can eat for 10 cents" trial sale at his walk-in ice cream store. Within two hours he had dished out more than 1,600 servings in Moorhead, North Dakota.
The first stpre was in Joliet, Illinois, at 501 N. Chicago Street. The building still stands, but has not been used as a Dairy Queen since the 1950s.