Airline flights began by 1939, on Mid-Continent. Successor Braniff pulled out in 1959, replaced by Frontier Airlines and Ozark Air Lines; Ozark left in 1960 and Frontier left in 1969, three years before Kansas City International Airport opened south of St Joseph. Since then the airport has been used for crop spraying, charter passenger and cargo services, pilot training, nightly mail delivery, and until its acquisition by American Airlines, pilot training by Trans World Airlines (TWA). Its main use now is as a base for the Missouri Air National Guard.