See main article: Commencement Bay-class escort carrier.
In 1941, as United States participation in World War II became increasingly likely, the US Navy embarked on a construction program for escort carriers, which were converted from transport ships of various types. Many of the escort carrier types were converted from C3-type transports, but the s were instead rebuilt oil tankers. These proved to be very successful ships, and the, authorized for Fiscal Year 1944, were an improved version of the Sangamon design. The new ships were faster, had improved aviation facilities, and had better internal compartmentation. They proved to be the most successful of the escort carriers, and the only class to be retained in active service after the war, since they were large enough to operate newer aircraft.
Rabaul was long overall, with a beam of at the waterline, which extended to at maximum. She displaced 21397LT at full load, of which could be fuel oil (though some of her storage tanks were converted to permanently store seawater for ballast), and at full load she had a draft of . The ship's superstructure consisted of a small island. She had a complement of 1,066 officers and enlisted men.
The ship was powered by two Allis-Chalmers geared steam turbines, each driving one screw propeller, using steam provided by four Combustion Engineering-manufactured water-tube boilers. The propulsion system was rated to produce a total of for a top speed of 19kn. Given the very large storage capacity for oil, the ships of the Commencement Bay class could steam for some 23900nmi at a speed of .
Her defensive anti-aircraft armament consisted of two dual-purpose guns in single mounts, thirty-six Bofors guns, and twenty Oerlikon light AA cannons. The Bofors guns were placed in three quadruple and twelve twin mounts, while the Oerlikon guns were all mounted individually. She carried 33 planes, which could be launched from two aircraft catapults. Two elevators transferred aircraft from the hangar to the flight deck.
The first fifteen ships of the Commencement Bay class were ordered on 23 January 1943, allocated to Fiscal Year 1944. The keel for Rabaul was laid down at the Todd Pacific Shipyards in Tacoma, Washington on 29 January 1945. She was named for the island of Rabaul in New Georgia, a major Japanese base during World War II, which was neutralized during a lengthy campaign in 1943 and 1944. The ship was launched on 14 June, and was delivered to the US Navy on 30 August, shortly after the end of the war.
The ship was not commissioned, and was instead assigned to the Pacific Reserve Fleet, based in Tacoma. Ten of the Commencement Bay-class ships saw significant service postwar as anti-submarine warfare (ASW) carriers, but they were small and had difficulty operating the new Grumman AF Guardian patrol planes, so the rest of the class remained laid up, and they were soon replaced in the ASW role by much larger s. She was reclassified as a helicopter escort carrier, with the hull number CVHE-121, in June 1955. Three years later, in June 1958, she was moved to the San Diego Group of the Pacific Reserve Fleet.
In May 1959, Rabaul was reclassified as an aircraft ferry, with the hull number AKV-121. In the mid-1960s, the US Army explored the concept of a Floating Aircraft Maintenance Facility (FAMF), and in September 1967, the Department of Defense selected Rabaul to be converted for FAMF II, but the project ultimately came to nothing. She remained part of the reserve fleet until she was struck from the Naval Vessel Register on 1 September 1971. The ship was eventually sold for scrap on 25 August 1972 to the Nicholai Joffe Corporation, based in Beverly Hills, California. Shortly before scrapping, she was used in the closing scenes of the 1973 movie Magnum Force.