For security reasons, the Pentagon forbids camera crews near the entrances to the complex. John Frankenheimer wanted a shot of Kirk Douglas entering the building. So they rigged up a station wagon with a camera to film Douglas, in a full Marine colonel's uniform, walking up the steps of the Pentagon. The salutes Douglas received in that scene were real, as the guards had no reason to believe it was for a movie!
Kirk Douglas had originally signed to play Gen. James Mattoon Scott. Douglas eventually realized that his friend Burt Lancaster would be ideal as Scott, and took the less flashy role of Col. Martin "Jiggs" Casey after Lancaster signed on to the film.
Some film reference works (e.g., the multivolume set, "The Motion Picture Guide") incorrectly list Jack Mullaney's character as "Lt. Hough". "Hough" is the last name of this character in the novel upon which the film is based.
Col. Casey's first name, Martin, is never spoken; he is always addressed (or referred to) by his nickname, "Jiggs". Casey's full name can be seen on the window that separates his office from the waiting room outside General Scott's office.
The "Eleanor Holbrook" subplot was based on a real-life incident involving General Douglas MacArthur. In 1934, the general sued journalists Drew Pearson and Robert Allen for libel. He dropped the suit when the defendants announced they intended to take testimony from Isabel Rosario Cooper, a Eurasian woman who had been the general's mistress.
This movie was never released in Brazil, due to the "coup-d'etat" organized by the military (1 April 1964). The plot and real life were too close.
Paul Girard (Martin Balsam) meets Admiral Barnswell (John Houseman), commander of the 6th Fleet, in Gibraltar aboard his flagship, USS Kitty Hawk, one of the newest & largest aircraft carriers in 1964. The scene was filmed in San Diego Bay, where the Kitty Hawk was actually flagship of the 7th Fleet based in the Pacific. The aircraft carrier USS Midway is in the background. The Midway is now a museum in San Diego while the Kitty Hawk is still in service (2008) and the longest serving US Navy ship.
John Frankenheimer had been in the Air Force and was very familiar with the Pentagon.
During a briefing between Col. "Jiggs" Casey and Gen. Scott in Scott's Pentagon office, the second shot on the video screen, allegedly of B-47s taxiing at Wright Field during the January alert, is footage from the film Strategic Air Command (1955).
In the original novel upon which the film was based, Admiral Farley C. Barnswell's flagship is the USS Eisenhower, a good guess on the part of the novelists Fletcher Knebel and Charles W. Bailey II: an aircraft carrier of that name would not be ordered until 29 June 1970, a full eight years after the book's publication.