Final answer:
The first Dairy Queen store opened in 1940 in Joliet, Illinois, becoming a significant part of American fast food history.
Step-by-step explanation:
The first Dairy Queen store open was in 1940 in Joliet, Illinois, USA. This became a landmark event in American fast food history, where the Dairy Queen brand became known for its soft serve ice cream and later, a wide variety of frozen treats and fast food. The concept of frozen dairy treats served in a retail setting was relatively new at the time, and Dairy Queen became one of the pioneers in this sector.