The Hughes Tool Company series (1912-1990) encompasses the administrative, financial, and legal management of Hughes Tool Company. Materials within the series depict Howard Hughes' control of the company; his film, aeronautics, electronics, and real estate ventures; and his lawsuits and United States Senate hearing. The series also includes research reports on the corporate history of Hughes Tool Company and its subsidiaries, copyright histories for films Hughes produced, and correspondence from Hughes' associates searching for reels of Hughes' films domestically and internationally.

Materials contain photographs of the HK-1 Hercules, otherwise known as the "Spruce Goose" or the "Flying Boat," from 1942 to 1947. The photographs primarily depict the construction, transportation, and storage of the plane, but also include photographs of the first and only test flight of the HK-1 above Los Angeles Harbor in 1947. Howard Hughes designed the HK-1 as the world's largest plane, capable of transporting large quantities of U.S. military hardware and personnel. In 1947, under the program's new designation H-4 Hercules, Hughes had the plane transported from his factory in Culver City, California to Los Angeles Harbor. On November 2, he piloted the plane during its only test flight. The U.S. Air Force abandoned the controversial project, and Hughes was called to testify before the Truman Committee of the U.S. Senate to justify the use of government funds on a program that never succeeded.