On Labor Day, September 6, 1937, the National Air Races were held in Cleveland, Ohio. As can be seen in the following New York Times listing, the start and finish of the race were broadcast.
No references to Stewart being a part of this broadcast have been discovered. However, the following article appeared in Stewart’s hometown newspaper, the Indiana Evening Gazette, a few days after the races.
Based on this article, Jimmy was at the races and was part of a broadcast. Whether that appearance was part of the national hook-up or just an interview on a local station, is not known.