Universal Pictures bought Clair Huffaker’s original screenplay, The Hellfighters, in early 1967, as announced in the 17 Feb 1967 DV and 20 Feb 1967 LAT. Robert Arthur was assigned by the studio to produce. Later that year, the 7 Dec 1967 DV reported that Andrew V. McLaglen had been hired to direct, with John Wayne set to star. The production budget was cited as $6 million in an 8 Jun 1968 LAT article.
       Principal photography began 14 Mar 1968, as stated in a 15 Mar 1968 DV production chart. Location filming took place in Houston, TX, according to items in the 20 and 21 Mar 1968 DV. Inclement weather delayed exterior filming at an oil field, but McLaglen kept on schedule by shooting interior scenes at a covered set in a warehouse. Cast and crew planned to move to Jackson Hole, WY, on 22 Apr 1968, as noted in a 19 Apr 1968 DV brief. By mid-May, production had moved again to Casper, WY, where an oil field fire sequence was shot in the absence of three technical advisors – “Red” Adair, “Boots” Hansen, and “Coots” Matthews – who were called away to a “wildcat well blowout in Patterson, LA,” and a fire in Bolivia, as reported in the 16 May 1968 DV. On location in Casper, a catering truck overturned onto John Wayne’s dressing room trailer, smashing in windows and bending its steel frame, as reported in the 25 May 1968 LAT. Wayne and his makeup man escaped uninjured.
       Jill Donohue, Charles Brewer, and Pedro Gonzalez-Gonzalez were added to the cast during filming, according to items in the 15 Mar 1968 LAT, 3 May 1968 DV, and 15 May 1968 Var. The 20 Mar 1968 DV stated that Wayne and co-star Bruce Cabot hoped to convince actress Gene Tierney, then living in Houston, to play a cameo.
       Although Katharine Ross was a relative newcomer to feature films, she was set to receive equal, “same size” billing alongside John Wayne’s name in advertisements and screen credits. The 20 Mar 1968 DV cited her recent Academy Award nomination for her role in The Graduate (1967, see entry) as a contributing factor to the terms of her billing.
       Hellfighters premiered in Houston on 19 Dec 1968. The 4 Dec 1968 Var noted Universal’s plans to release the picture in over 200 theaters in TX and surrounding states, to gain good word-of-mouth before opening in other parts of the country.