The working title of this film was Son of Flicka, and some contemporary sources referred to it as Thunderhead. Although the film's title contains no punctuation in the onscreen credits, it is most commonly referred to as Thunderhead, Son of Flicka by contemporary sources. The film's opening title card reads, "Twentieth Century-Fox presents Mary O'Hara's Thunderhead Son of Flicka." After the opening credits, the following written prologue appears: "The producers are grateful for the invaluable assistance and approval of the American Humane Association under whose supervision this picture was made." Mary O'Hara's novel was serialized in McCall's (Jul--Sep 1943).
       The picture is a sequel to the 1943 production My Friend Flicka, and according to information in the Twentieth Century-Fox Records of the Legal Department and the Produced Scripts Collection, located at the UCLA Arts--Special Collections Library, when the studio purchased the rights to O'Hara's novel My Friend Flicka, it reserved the right to make a sequel of its own creation, using O'Hara's characters. Originally, the studio intended to feature the characters in a new film based on the Will James's story Smoky, which had previously been produced by the company in 1933 (see the entry for Smoky in the AFI Catalog of Feature Films, 1931-40; F3.4162). The studio changed course and purchased the rights to O'Hara's novel Thunderhead after experiencing difficulties with the screenplay and receiving a number of complaints from O'Hara, who stated that she was having problems selling the rights to her novel because other companies assumed that Twentieth Century-Fox had already purchased it. The studio did produce another version of Smoky, which was released in 1946 (see above).
       According to a Nov 1943 HR news item, Peggy Ann Garner was to be included in the cast, and Ruth Clifford tested for a role. Various HR news items list Archie Mayo, John Brahm, Irving Pichel and Lew Seiler as potential directors for the picture.
       The film was shot on location at various sites, including the following Oregon locations: Bridal Veil Falls at Oneonta Gorge and the Multnomah County Fairgrounds near Gresham. California locations included Brent's Crags, Hidden Valley and Hollywood Park Racetrack. In Utah, the company filmed at: Zion National Park, Kanab, Bryce Canyon National Park, Red Rock Canyon, Navajo Lake, Glendale Gorge, Cedar City and Cedar Breaks National Monument. Additional scenes were shot in Duck Creek, Nevada. Thunderhead, Son of Flicka was the first picture to be shot completely with Monopack, a new type of color film stock. The one-strip stock aided production because, unlike Technicolor's three-strip stock, it could be run through an ordinary black-and-white film camera instead of the bulky three-strip Technicolor cameras, which made location shooting especially difficult.
       On 25 Feb 1946, McDowall, Foster and Johnson appeared on a Lux Radio Theatre broadcast of the story. The first film to feature the "McLaughlins" was My Friend Flicka, and the third picture, entitled Green Grass of Wyoming, was released by Twentieth Century-Fox in 1948 (see above). Although "Flicka's" sire is identified as "Banner" in My Friend Flicka, in both Thunderhead, Son of Flicka and Green Grass of Wyoming, her sire is the "Albino."