Although the plot is mostly fiction, Wyatt Earp and Tom Mix were real-life friends. After Earp retired from law enforcement, he and his wife drifted around, eventually setting in Los Angeles, where he and Mix met. Mix tried to get Hollywood to produce a movie about Earp, but they weren't interested. Earp was hired as a technical consultant for movies starring Mix and William S. Hart. When Earp died in 1929, Mix and Hart were his pallbearers.
According to "James Garner: You Ought to be in Pictures", Garner said, "I'd never work with Bruce Willis again. I did that Blake Edwards film with him, Sunset (1988). Willis is high school. He's not that serious about his work. He thinks he's so clever he can just walk through it, make up dialogue and stuff. I don't think you work that way".
Wyatt Earp died on January 13, 1929, a few months shy of his 81st birthday. James Garner was 60 in 1988. Garner previously played Earp in Hour of the Gun (1967).
When Tom Mix and Wyatt Earp land the plane, they're met by two cowboys. The one on the left of the screen, Cowboy Henry, is Jack Garner, James Garner's real-life brother.