The Lockheed S-3 Viking was designed to meet a 1968 Navy specification for a replacement for the Grumman S-2 Tracker anti-submarine search and strike aircraft. With a crew of four, the S-3A was designed to carry a comprehensive array of avionics, as well as up to 60 sonabouys. The first S-3A flew on November 8, 1971, with the first carrier landings being made in November 1973.

     In 1980 Lockheed received a Navy contract for a weapons system improvement programme (WSIP), and in August 1981 full-scale development was launched, leading to two prototype conversions to S-3B configuration.  The improvements were related to avionics, including sonobuoy telemetering.  Provision was also made for the carriage of AGM-84A Harpoon anti-ship missiles, one on each wing, in place of the long-range tanks.  The first S-3B flew on September 13, 1984, and the production of kits for the conversion of all S-3As to the new standard began in 1986.
(source: United States Navy Aircraft since 1911 by Gordon Swanborough and Peter Bowers)