<p>While the R-6 was being developed, work was also in progress on a completely new helicopter, the VS-337, with an all-metal fuselage which provided tandem accommodation for two. Four prototypes, plus one additional at a later date, were ordered by the USAAF, and the first of these, with the designation XR-5 and powered by the 450-hp (336-kW) Pratt & Whitney R-985-AN-5 engine, was flown at Bridgeport, Connecticut on 18 August 1943. Total production of 65 included the remaining four XR-5s (two of which were fitted with British equipment and redesignated XR-5A), 26 YR-5As built for USAAF evaluation and 34 production R-5As. The last could be fitted with a litter carrier on each side of the fuselage and the type was used by the Air Rescue Service. The addition of a nosewheel, a rescue hoist, an external auxiliary fuel tank and a third seat identified the R-5D, 21 of which were modified from R-5As. For training duties, five YR-5As were fitted with dual controls and redesignated YR-5E.
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<p>A four-seat version was developed for the civil market as the S-51, the prototype making its first flight on 16 February 1946. Certification was achieved during the following month and initial deliveries were made in August. Los Angeles Airways used the S-51 to open the world's first scheduled helicopter mail service on 1 October 1947.
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<p>A total of 379 S-51s was built, many of them for the US armed forces. Eleven were acquired by the USAF in 1947 as R-5Fs (redesignated H-5F in June 1948), supplemented by 39 H-5Gs with a rescue hoist, for use by the Air Rescue Service, and USAAF procurement was completed by 16 H-5Hs purchased in 1949.
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<p>Following trials with two US Army R-5As, which were redesignated HO2S-1, the US Navy ordered a number of HO3S-1s which were equivalent to the USAF R-5F, deliveries commencing in November 1946.
<item><hi format=bold>Performance:</hi> maximum speed 106 mph (171 km/h); cruising speed 85 mph (137 km/h) at 1,000 ft (305 m); service ceiling 14,400 ft (4 390 m); range 360 miles (579 km)
<item><hi format=bold>Weights:</hi> empty 3,780 lb (1,715 kg); maximum take-off 4,825 lb (2,189 kg)
<item><hi format=bold>Dimensions:</hi> main rotor diameter 48 ft 0 in (14.63 m); length 57 ft 1 in (17.40 m); height 13 ft 0 in (3.96 m); main rotor disc area 1,810 sq ft (168.15 m<su>2</su>)
