Originally designated the XTB3F-1 (G-70), this aircraft was designed as a torpedo bomber with a composite power plant consisting of an R-2800-34W piston engine in the nose and a Westinghouse 19XB-2B turbojet engine in the rear fuselage. During evaluation trials considerable modifications were made to change its role to anti-submarine: in 1949 two new prototypes incorporating all the changes were built (carrying the designations XTB3F-1S and XTB3F-2S) the former equipped as a submarine hunter and the latter as a killer.

Both types were ordered by the US Navy under the respective designations AF-1S and AF-2S, but before completion of the first AF-1S its designation was changed to AF-2W. The AF-2S Guardian (G-82) carried one 900kg torpedo, two 720kg depth charges or two 900kg bombs internally, while a similar load could be carried externally. In addition a searchlight was carried under the port wing and a radar scanner under the starboard. The AF-2W carried a large search and early-warning radome beneath the forward fuselage. In 1952-53 the AF-3S version was produced with additional submarine detection gear. Production of the Guardian ended in March 1953.