The Westland Wessex was a license produced version of the Sikorsky S-58 which had seen extensive US service with the Army, Navy, and Marine Corps. It was based on the earlier S-55 (which Westland also manufactured) and was characterized with a large angular mounting in the nose for the twin radial engines which produced virtually twice as much power as its predecessor and which drove a four-blade main rotor. For shipboard stowage, the main rotor blades and rear section of the fuselage could also be folded making it an effective platform for anti-submarine warfare but also as a utility and transport helicopter. In British service, the Wessex featured a revised powerplant and various other improvements such as greater reliability, easier maintenance, and lower fuel consumption. Like their US counterparts, they were found mainly in naval roles but were also employed as troop transports and were only retired incredibly until 2003. Foreign operators of the Wessex included Australia, Iraq, Ghana, and Brunei.

The S-58 prototype, designated initially by the US Navy as the XHSS-1, first flew on 8 March 1954 entering service as the HSS-1 (later the SH-34G). The first version for the RN was the Wessex HAS.1 which featured a revised nose profile to fit the new Gazelle engine. It was followed by the HC.2 tactical transport for the RAF, the HAS.3 anti-submarine version with an upgrated powerplant, and ultimately the HU.5 used for a number of utility roles on the RN's commando carriers. A specialized VIP transport for service with The Queen's Flight was known as the HC.4 (and which were otherwise similar to the HC.2) while the Mk. 60 was a civil version used for search and rescue (SAR) or as an air ambulance.