[2] The limitations of the Regulus I were well known by the time it entered service in 1955, so the Navy issued a specification for a surface-launched supersonic shipborne cruise missile, equipped to carry a nuclear warhead, that had greater range, accuracy and resistance to countermeasures.

Development of the Regulus II was well under way when the program was canceled in favor of the UGM-27 Polaris SLBM (Submarine-Launched Ballistic Missile) system, which gave unprecedented accuracy as well as allowing the launch submarine to remain submerged and covert.

[1] The major drawback of the original Regulus was the use of radio-command guidance, which required a constant radio link with the launch ship / submarine that was relatively easy to interfere with.

[2] Prototype missiles were built, designated XRSSM-N-9 Regulus II, with retractable landing gear, to allow multiple launches, and Wright J65-W-6 engines and Aerojet General booster, which restricted them to subsonic flight.

Beginning in 1958, testing was carried out with the XRSSM-N-9a, equipped with the General Electric J79-GE-3 turbojet and a Rocketdyne solid-fueled rocket booster to allow the entire flight envelope to be explored.

[2] The airframe followed contemporary aircraft construction techniques, with weight savings from the use of advanced materials and the short airborne life of the missile.

Upon completion of the testing in Puerto Rico in 1963, the Regulus II drones were moved to NAS Point Mugu, California, where they remained in use until December 1965.