Dean Martin later said of John Wayne, "Someone else would have laid around, feeling sorry for himself, for a year. But Duke, he just doesn't know how to be sick. He's recuperating the hard way. He's two loud speaking guys in one. Me, when people see me, they sometimes say, 'Oh, there goes Perry Como.' But there's only one John Wayne, and nobody makes any mistakes about that".
This movie marked the return of John Wayne to work after having a cancerous lung and two ribs removed just four months earlier. He insisted on doing some of his own stunts to show the public that the illness hadn't slowed him down.
Filming was due to begin in October 1964, but had to be delayed until January 1965 after John Wayne was diagnosed with lung cancer.
The gun battle was loosely based on the real-life story of the five Marlow brothers. These were five brothers whose parents had established a homestead in Oklahoma territory, The father was a doctor and the sons raised horses that they sold to the army. Four of the brothers were falsely accused of horse theft, were arrested and brought to Graham, Texas. The remaining brother arrived in hope of proving their innocence, but he was arrested too. After several attempts by a mob to lynch them, they were chained together and were being transported to Weatherford, Texas for trial. At Dry Creek, at the edge of the town of Graham, the mob attacked them. The brothers took the deputies' guns and a battle ensued, much as was portrayed in this film. Two of the brothers were killed. The others escaped. One was later poisoned and then shot in an attempt to make it seem he was shot avoiding capture. The killer was himself charged with murder. In 1891, during the trial of members of the mob, the judge had high praise for the Marlows for their courage and boldness, saying that it would be remembered in story and song. The remaining two brothers moved to Colorado and became well-respected lawmen.
When John Wayne is dragged into the river, a child calls out, "Come on Dad!" This was his three-year-old son Ethan Wayne, who was watching off camera and knew how ill his father was. Wayne contracted a serious cold due to filming this scene, and narrowly avoided the cold worsening into full blown pneumonia.