Last in the line of Lockheed twin-engined commercial transports, the Model 18 Lodestar was developed from the Model 14 and the prototype, flown for the first time on 21 September 1939, was a conversion from a standard Lockheed 14-H2. With a longer fuselage seating 14 passengers and having a crew of three, the Lodestar was built in a number of versions, principally the Models 18-07 with 750-hp (559-kW) Pratt & Whitney S1E-3G Hornet engines, 18-08 with 900-hp (671-kW) Pratt & Whitney SC-3G Twin Wasps, 18-14 with 1,050-hp (783-kW) S4C-4G Twin Wasps, 18-40 with 900-hp (671-kW) GR-1820-G102A Wright Cyclones, 18-50 with 1,000-hp (746-kW) GR-1820-G202A Cyclones, and 18-56 with similarly powered GR-1820-G205A Cyclones.

US military interest in the Lodestar was first shown in 1940, when the US Navy ordered a single XR5O-1 and two R5O-1 command transports, a similar aircraft being delivered to the US Coast Guard. These were powered by Wright R-1870 engines, as were 12 R50-4s, 41 R50-5s and 35 R50-6s. These last three models were, respectively, four- to seven-seat executive transports, 12/14-seat personnel transports, and 18-seat troop carriers used by the US Marine Corps for paratroop operations. Pratt & Whitney R-1830 engines powered the single R50-2 and three R50-3s built for the US Navy.

The US Army Air Corps ordered a single Wright R-1820-29-engined aircraft in May 1941, under the designation C-56, this being the military version of the civil Model 18-50. At the same time three Model 18-14s were ordered, with Pratt & Whitney R-1830-53 engines. These were supplemented by later orders for seven and three aircraft, respectively, all 13 machines being designated C-57.

An impressed civil aircraft was designated C-57A, seven troop carriers were known as C-57Bs and three of the later model C-60As were re-engined with Pratt & Whitney R-1830-43 radials to become C-57Cs; one of these three became a C-57D with R-1830-92 engines.

The greater number of aircraft taken over from the US internal airlines from December 1941, however, were given designations in the C-56 series, comprising one C-56A, 13 C-56Bs, 12 C-56Cs, seven C-56Ds and two C-56Es.

Ten Model 18-07s and 15 Model 18-56s were acquired under the Defense Aid program as C-59s and C-60s, respectively, later supplemented by another 21 C-60s and 325 C-60As, one of the latter becoming a C-60B with an experimental hot-air de-icing system. A single Model 18-10, with 1,200-hp (895-kW) R-1830-53 engines and seats for 11 passengers, was purchased in 1942 and designated C-66.