The SSM-N-8A Regulus, also known as the Regulus I and RGM-6, was a nuclear-capable turbojet-powered second generation cruise missile operated by the US Navy between 1955 and 1964. Its development was an outgrowth of U.S. Navy tests conducted with the German V-1 missile at Naval Air Station Point Mugu in California. Its barrel-shaped fuselage resembled that of numerous fighter aircraft designs of the era, but without a cockpit. Test articles of the Regulus were equipped with landing gear and could take off and land like an airplane. When the missiles were deployed they were launched from a rail launcher, and equipped with a pair of Aerojet 2.2-KS-33,000 JATO bottles on the aft end of the fuselage.

Nazi Germany's use of the V-1 flying bomb during World War II marked the first combat deployment of a cruise missile, highlighting the potential of a new class of weapon. Even before Germany's surrender, the United States captured, reverse-engineered, and mass-produced its own version of the V-1, the Republic-Ford JB-2 Loon, intended for use against Japan. With the war's end and the onset of the Cold War, the U.S. sought new ways to deploy nuclear warheads. One proposal, put forward by Captain Thomas Klakring, was to launch nuclear missiles from submarines. He argued that submarines would be far more difficult to detect and attack than surface ships, such as aircraft carriers, while also introducing a new method of warhead delivery beyond aircraft. At the time, bombers were the U.S. military's only means of delivering nuclear warheads, a reliance that threatened to diminish the Navy's role and overemphasize a single approach to nuclear strategy.

Klakring proposed launching the Loon from submarines to test his concept. Work began in 1946, and by 1947, USS Cusk became the first U.S. submarine to fire a guided missile. Testing continued for several years, but significant issues arose. Cusk nearly sank when a Loon exploded on deck, and the missile itself was slow, had limited range and payload, and was impractical for military use. Despite these shortcomings, the tests successfully demonstrated how a submarine could surface, launch a missile, and submerge again, making it difficult for an enemy to retaliate. The Navy acknowledged the concept’s potential but recognized the need for a missile specifically designed for submarine deployment.

Development of such a missile began with Grumman's SSM-N-6 Rigel and the Johns Hopkins Applied Physics Laboratory's SSM-N-2 Triton. However, work did not begin in earnest until August of 1947, days after the United States Army Air Force awarded a contract for the ground-based MGM-1 Matador cruise missile. The Navy, not wanting to lose its edge in missile development, began Project Regulus with the same equipment used in the Matador. On 17 November 1947, a contract was awarded to Chance Vought, which previously investigated a similar project as part of a study on "pilotless missiles". The company, desperate for government contracts as the money-making F4U Corsair production line was nearing an end, focused on a practical design that could be brought into service quickly. With its survival at stake, the company invested heavily in research and development, drawing from experience with jet fighter projects such as the broadly similar F6U Pirate. The Navy initially planned on having each missile be operational in a sequential order, starting with the simple Regulus and ending with the more complex but capable Triton in 1960. However, both the Rigel and Triton were powered by ramjets, which presented a significant challenge as the technology was too immature for immediate use, eventually leading to both programs' cancellation.

The contract required the missile to have a range of 500 nautical miles (930 km) at Mach 0.85, a 3,000-pound (1,400 kg) warhead, and a circular error probable (margin of error) of 25 nautical miles (46 km). Regulus was designed to be 30 feet (9.1 m) long, 10 feet (3.0 m) in wingspan, 4 feet (1.2 m) in diameter, and would weigh between 10,000 and 12,000 pounds (4,500 and 5,400 kg). The missile somewhat resembled the contemporary F-84 Thunderjet fighter aircraft, but without a cockpit, and test versions were equipped with landing gear so that they could be recovered and re-used. After launch, Regulus would be guided toward its target by control stations, typically by submarines or surface ships equipped with guidance equipment. It could also be flown remotely by chase aircraft. (Later, with the "Trounce" system (Tactical Radar Omnidirectional Underwater Navigational Control Equipment), one submarine could guide it). Army-Navy competition complicated both the Matador's and the Regulus' developments. The missiles looked alike and used the same engine. They had nearly identical performances, schedules, and costs. Under pressure to reduce defense spending, the United States Department of Defense ordered the Navy to determine if Matador could be adapted for their use. The Navy concluded that the Navy's Regulus could perform the Navy mission better.

Regulus had some advantages over Matador. It required only two guidance stations while Matador required three. It could also be launched quicker, as Matador's boosters had to be fitted while the missile was on the launcher while Regulus was stowed with its boosters attached. Finally, Chance Vought built a recoverable version of the missile, designated KDU-1 and also used as a target drone, so that even though a Regulus test vehicle was more expensive to build, Regulus was cheaper to use over a series of tests. The Navy program continued, and the first Regulus flew in March 1951.

Due to its size and regulations concerning oversize loads on highways, Chance Vought collaborated with a firm that specialized in trucking oversize loads to develop a special tractor trailer combination which could move a Regulus I missile.

The first launch from a submarine occurred in July 1953 from the deck of USS Tunny, a World War II fleet boat modified to carry Regulus. Tunny and her sister boat USS Barbero were the United States's first nuclear deterrent patrol submarines. They were joined in 1958 by two purpose-built Regulus submarines, USS Grayback and USS Growler, and, later, by the nuclear-powered USS Halibut. Halibut, with its extremely large internal hangar could carry five missiles and was intended to be the prototype of a whole new class of cruise missile firing SSG-N submarines.