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The film's opening title card reads "Twentieth Century-Fox Presents Zane Grey's Western Union." Although some modern sources assert that Grey did not write a novel entitled Western Union, and claim that the film was instead based on an original story by studio writers, the Twentieth Century-Fox Records of the Legal Department, located at the UCLA Arts-Special Collections Library, indicate that Grey did write the novel, which was published on October 20, 1939, three days before his death. Both Grey's novel, his last, and the film present a fictional account of the real Edward Creighton, a Western Union engineer who helped survey and build the telegraph line from Omaha, NE to Salt Lake City, UT. A January 26, 1941 New York Herald Tribune article commented about the film: "In 1861 it cost $212,000 to extend the telegraph from Omaha to Salt Lake City, and the crew took four months and eleven days, covering 1,100 miles, to do the job. To reproduce their feat in 1940, a company of 300 traveled 2,000 miles in ten months at a cost of more than $1,000,000."
According to a October 26, 1939 Hollywood Reporter news item, Paramount attempted to acquire rights to Grey's novel. Grey's son, Romer, reportedly revealed that before his father's death, "negotiations had been on for Gary Cooper to star in the production which [Grey] was to produce for either United Artists or RKO release." Twentieth Century-Fox obtained the rights to Grey's novel in November 1939 for $25,000, and according to a Hollywood Reporter news item, the studio also purchased an original story by Ward Wing about the history of the Western Union company. Apparently the studio did not intend to use Wing's material, but purchased it "to forestall any conflict with the Zane Grey yarn." A modern source states that executive producer Darryl F. Zanuck originally considered assigning Irving Pichel as the film's director. Studio legal records indicate that writers Albert Shelby LeVino, Curtis Kenyon and Kenneth Earl worked on early drafts of the film's screenplay, but the extent of their contribution to the completed picture has not been confirmed.
A August 12, 1940 Hollywood Reporter news item announcing the signing of Fritz Lang as director also noted that Brenda Joyce had been cast in the feminine lead, and Don Ameche and Lloyd Nolan had been assigned to leading roles. On September 11, 1940, however, Hollywood Reporter noted that Twentieth Century-Fox was borrowing Robert Young from M-G-M to replace Ameche. Other Hollywood Reporter news items stated that Laird Cregar would be in the cast, then announced that because he was being held up by his work in Hudson's Bay, he was to be replaced by George "Gabby" Hayes, but illness prevented Hayes from appearing in the finished film. Lucille Miller and Esther Brodelet were included in the cast by Hollywood Reporter news items, but their participation in the completed film has not been confirmed. A Hollywood Reporter news item also included Mary Astor in the cast, but she does not appear in the released picture. Actor Chill Wills was borrowed from M-G-M for the production.
A October 21, 1940 Hollywood Reporter news item noted that during principal photography, the film's script was sent "back to writer George Bruce with instructions to build up the part of Virginia Gilmore." The extent of Bruce's contribution to the released film has not been determined, however. The picture was largely shot on location at Kanab and Zion National National Park, UT, and House Rock Canyon, AZ. According to a September 26, 1940 Hollywood Reporter news item, fourteen Native Americans traveled from Hollywood to the Kanab location because "any arrangements for use of government reservation Indians in that territory [would involve] too much 'red tape.'" A December 1, 1940 New York Times article reported that Lang did not cast the local Piute Indians in the picture "because of their stature [which meant that] they didn't look like the customers' conception of Indians." The article stated that instead, Lang "ordered a shipment of Hollywood Indians from Central Casting-tall, high cheek-boned fellows who look like aborigines are supposed to look." According to a studio press release, Twentieth Century-Fox actor Henry Fonda, who had once worked as a lineman and in telegraph laboratories in Minneapolis, served as a technical advisor on the production.
According to a Hollywood Reporter news item, in December 1940, the studio purchased J. Hyatt Downing's novel Sioux City, intending to produce it as a "follow-up" to Western Union. Randolph Scott and Laird Cregar were to star in the picture, but it was never made.