Douglas released an extended-range variant of the DC-7 in 1956. Named the DC-7C (Seven Seas), it offered very long range, non-stop international service. Two 5 ft wingroot inserts added fuel capacity, reduced interference drag and made the cabin quieter by moving the engines farther outboard. All DC-7Cs also had fuel tanks in the engine nacelles. Compared to the DC-6B, a 40 in plug ahead of the wing and a 40-inch plug behind the wing stretched the DC-7C fuselage to a total length of 112 ft 3 in.
Northwest Airlines took advantage of the DC-7Cs long range by offering scheduled service from the continental U.S. to Japan. Called Imperial Service, it was billed as “the most gracious experience in the history of flight”