The DC-6 was developed from the DC-4 to compete with the Boeing Stratoliner and Lockheed Constellation, both of which were pressurized. Douglas was able to capitalise on the Air Force’s desire to develop an improved version of the C-54 by persuading the Government to fund construction of the XC-112A. Although funding dried up with the end of World World II, Douglas had already received an order from American Airlines for 50 DC-6s. An order from United Airlines followed soon after, and the two airlines received the first deliveries simultaneously. A handful of DC-6s remain commercially active, in Alaska; although overall survivor numbers have dipped below 100 for the first time.
First flight: 15 Feb 46 (c/n 36326, 45-873)
Production: 704, at Santa Monica, CA
First delivery: 20 Nov 46, to American Airlines (c/n 42855, N90702) and United Airlines (c/n 42865, N37501)
Last delivery: 17 Nov 58, to Jugoslovenski Aero Transport (c/n 45564, YU-AFB)
Variants: XC-112A - prototype, derived from the C-54G but powered by 4 Pratt & Whitney R-2800-34 Double Wasp radials, featuring stretched fuselage, pressurized cabin, increased accommodation and payload, and performance improvements (1 built);
DC-6 - initial production version powered by R-2800-CA15s, with seating for up to 86 passengers (175 built, including one for the USAF as a Presidential transport under the designation C-118; first flight 29 Jun 46);
DC-6A - stretched version powered by R-2800-CB16 or -CB17 radials, optimized for all-cargo use with strengthened floor, two large cargo doors, increased fuel capacity and payload of 28,000 lbs (74 built, including some as DC-6C with provision for rapid conversion between passenger and all-cargo configurations);
DC-6B - DC-6A with cargo modifications deleted and accommodation for up to 102 passengers (288 built);
C-118A Liftmaster - military version of the DC-6A, powered by R-2800-52W radials (101 built);
R6D-1 - US Navy version of C-118A (65 built, of which 40 transferred to the USAF as C-118A and surviving US Navy examples re-designated C-118B in 1962).
Conversions: Pacific Airmotive was licensed to convert DC-6Bs to DC-6As and there were many other conversions to all-cargo configuration under unofficial designations that included DC-6AB, DC-6AC and DC-6BF. In addition, Sabena converted 2 DC-6Bs into swing-tail cargo aircraft in 1968.