In 1971 James Keach and Stacy Keach played brothers in The Wright Brothers (1971). This gave James the idea they should play Jesse and Frank James in a movie together. Stacy financed James to write a country musical about the brothers which was eventually produced in Pennsylvania and New York. The Keaches then decided to turn the musical into a feature film screenplay in which both could star. In 1974 James was acting opposite Robert Carradine in The Hatfields and the McCoys (1975) and mentioned the project to him; Carradine suggested he and his brothers play the Younger brothers. The idea that all the brothers in the story be played by real-life brothers expanded.
The roles of Jesse James and his son, little Jesse, are played by father and son, James and Kalen Keach.
The film stars four sets of actual brothers: the Carradines, the Keachs, the Quaids and the Guests.
Edward Bunker who plays Chadwell in the Northfield bank robbery was actually, among many other things, a convicted bank robber.
Originally Jeff Bridges and Beau Bridges were going to play the Ford brothers but they could not fit it in their schedules.