In addition to his film career, Stewart had diversified investments including real estate, oil wells, the charter-plane company Southwest Airways and membership on major corporate boards, and he became a multimillionaire. Already prior to his enlistment in the Air Corps, he had been an avid amateur pilot, with a private pilot certificate and a commercial pilot license as well as over 400 hours of flying time. A highly proficient pilot, he entered a cross-country race with Leland Hayward in 1937, and was one of the early investors in Thunderbird Field, a pilot-training school built and operated by Southwest Airways in Glendale, Arizona.