Based on experience with the HSS-1 helicopters operating in anti-submarine hunter/killer pairs, the US Navy contracted on December 24, 1957 for development of a new helicopter in which the two roles could be combined. The ability to carry both search equipment and 840 lb of weapons, as well as instrumentation for all-weather operations, meant that a considerably larger helicopter was needed than the S-58 series already in use. A watertight hull design was used since most of the operational life would be spent over water, and stabilizing floats provided a convenient housing for the retractable wheels, making this helo a true amphibian.
(source: United States Navy Aircraft since 1911 by Gordon Swanborough and Peter Bowers)