Cold War, the
Soviet Navy built up a large and varied fleet of submarines which at one point was in excess of 200 operational submarines. The US Navy decided to counter this threat by the improvement and development of various
anti-submarine warfare (ASW) capabilities, which would result in the development of the Sea King.
 During the late 1950s, the US Navy was keen to take advantage of newly made progress upon
turboshaft engines by commissioning a sizable navalised helicopter for their purposes. Sikorsky received a request from the US Navy to design a new turbine-powered helicopter capable of performing the ASW mission; the specification provided included a dipping
sonar, a mission endurance of four hours, and the ability to support a weapons load of 840 lb.
In 1957, Sikorsky was awarded a contract to produce an all-weather
amphibious helicopter for the U.S. Navy. As per the earlier specification, this new helicopter was to excel at ASW; in particular, it would combine the roles of hunter and killer, as these duties had previously been carried out by two separate helicopters.
 It was also the first helicopter to be procured under the US Navy's new weapon system concept, under which Sikorsky was responsible not only for the design and production of the airframe itself but all of the major onboard systems, such as the
sonar, navigational equipment, electronic devices, and support equipment. As such, the navigation suite for the rotorcraft was developed jointly by Sikorsky and the US Navy.
XHSS-2 Sea King prototype
SH-3As of HS-6 above
in the early 1960s
Key features of the emerging ASW helicopter would include its
hull, enabling the rotorcraft to readily perform water landings, and the adoption of a twin-
turboshaft engine arrangement that enabled a larger, heavier and better-equipped aircraft than had been possible with prior helicopters.
 The designation HSS-2 was applied to imply a level of commonality to the earlier
HSS-1, should political sentiment turn against the development of an entirely new rotorcraft.
 A total of ten prototypes were produced to support the development program.
 In March 1959, the first prototype conducted its
In early 1961, a pair of prototypes were stationed on board the
Lake Champlain to fulfill a demand for carrier suitability trials to be conducted; these trials, which had involved testing the folding mechanism of the main
rotor blades and a series of takeoffs performed during winds of up to 50 MPH, were completed successfully in mid-1961.
 Shortly after the competition of suitability trials, the US Navy formally accepted delivery of the first HSS-2 rotorcraft, which would be subsequently re-designated as the SH-3A, in September 1961.
 Upon entering service, it was not only the largest amphibious helicopter in the world, but also held the distinction of being the first all-weather rotorcraft to reach production status for the US Navy.
In late 1961 and early 1962, a modified U.S. Navy HSS-2 Sea King was used to break the FAI 3 km, 100 km, 500 km and 1000 km helicopter speed records. This series of flights culminated on 5 February 1962 with the HSS-2 setting an absolute helicopter speed record of 210.6 mph.
 This record was broken by a modified French
Super Frelon helicopter on 23 July 1963 with a speed of 217.7 mph.
The base design of the Sea King had proved sound and several aspects were judged to be potentially useful for other operators, thus Sikorsky elected to pursue the further development of the Sea King for other markets beyond the US Navy.
 One of the major variants of the Sea King to be produced was a model for civil operators, which was designated as the
Sikorsky S-61L. The first operator of the S-61L was
Los Angeles Airways, who introduced the type to service on 11 March 1962.
 Another noteworthy Sea King variant, the significant change this time being the adoption of a conventional hull, the
Sikorsky S-61R, was also concurrently developed for transport and
search and rescue (SAR) duties, this type being extensively operated by the
U.S. Air Force and the
U.S. Coast Guard.
In US Navy service, the initial SH-3A model of the Sea King would be progressively converted into the improved SH-3D and SH-3H variants; these featured more powerful engines and improved sensors that gave the type greater operational capabilities as an ASW platform. It was also common for Sea Kings to be converted for non-ASW activities, these roles included minesweeping, combat search and rescue, and as a cargo/passenger utility transport.
 The aircrew on ASW-tasked Sea Kings were routinely trained to carry out these secondary roles as aircraft could often be quickly adapted to perform different missions in the face of operational needs.
Royal Canadian Navy (RCN) became a major operator of the type (see
Sikorsky CH-124 Sea King), the Sea King continues to operate as Canada's dominant maritime helicopter 50 years following its introduction to service in 1963.
 One notable innovation in Canadian operations, which was subsequently adopted by several other nations, was the use of a winch 'hauldown' landing method, referred to as a '
Beartrap'. This device considerably increased the ability of Sea Kings to land in difficult conditions, such as on small flight decks or during poor weather conditions.
In addition to aircraft manufactured by Sikorsky, several
license agreements were issued to other firms to produce the type, such as Mitsubishi in Japan and Agusta in Italy. Another licensee in the United Kingdom, Westland Helicopters, would substantially modify the Sea King, producing the
Westland Sea King.
 Unlike US Navy Sea Kings, the Westland Sea King was intended for greater autonomous operation.
 In total, Westland produced 330 Sea Kings; beyond British operators, export customers of Westland's Sea King included the
Indian Naval Air Arm, the
German Navy, the
Royal Australian Navy, and the
Royal Norwegian Air Force.
In the early 21st century, following their drawdown in US service, there have been a number of initiatives to refurbish ex-military Sea Kings for continued operations; in addition to civil operators, nations such as Egypt and India acquired refurbished former US Sea Kings to supplement their own aging fleets.
 While Sikorsky had ended production of the type during the 1970s, it was reported that nearly 600 Sea Kings were in operational service in 2009.