John Wayne, who had lost his entire left lung and several ribs in major surgery for cancer in 1964, had great difficulty breathing on an airplane while flying to the location for the start of filming and had to use an oxygen mask throughout the journey. Kirk Douglas recalled that he hadn't realized just how fragile Wayne was until this moment.
During the production, Kirk Douglas was late to the set because he was shooting a commercial endorsement for the Democratic Governor of California, Edmund G. Brown. John Wayne was furious, and was late to work the next day because he was shooting a commercial for the Republican candidate Ronald Reagan.
In the initial release cut of the film, when bare-chested Kirk Douglas walks away after John Wayne leaves his room, Douglas was nude. In other versions, the scene is cut just before Douglas' buttocks are revealed.
The War Wagon itself was built mostly of plywood and other lightweight materials, and painted to look like iron; appropriate "metallic" sound effects - such as when the "heavy" iron doors are opened and closed, etc. - were added to complete the illusion. For many years, at least through the 1980s, the deteriorating remains of the War Wagon were displayed in "The Boneyard" (a collection of old outdoor movie props) as part of the Universal Studios Backlot Tour in California.
According to the production notes on the 2003 DVD release, Keenan Wynn's battered hat that he wears in the picture was Leslie Howard's Confederate cavalry hat from Gone with the Wind (1939) which Wynn purloined from MGM. Wynn first wore the hat in a 1942 MGM screen test and "wore it in every picture he made".
Kirk Douglas was one of the few people in Hollywood who did not refer to John Wayne as "Duke", citing his dislike of nicknames. Curiously, Yakima Canutt, an expert stuntman and second unit director and one of Wayne's oldest friends in the business, also called him "John" in most print interviews.
The wagon itself, made mostly of gray painted plywood to resemble steel, with carriage bolts to look like rivet heads, was on display at Universal Studio in Orlando, Florida, during the 1990s. Interestingly, one hub on the wagon was originally on a piece of John Deere farm equipment, as evidenced by the John Deere name cast in the hub.
This movie was filmed in Durango, Mexico. The big rocks that you see is an area called "Los Organos" which translates The Organs, since they resemble organs. This area is vast and open to turists now. John Wayne filmed several movies here and also in a near town called "El Ojo de Agua" also in Durango know for its beautiful cascades.