 The destroyer escort concept came from design studies by the General Board of the United States Navy in 1940, as modified by requirements established by a British commission in 1941 prior to the American entry into the war, for deep-water escorts.The American-built destroyer escorts serving in the British Royal Navy were rated as Captain-class frigates.The U.S. Navy's two Canadian-built and 96 British-influenced, American-built frigates that followed originally were classified as "patrol gunboats" (PG) in the U.S. Navy but on 15 April 1943 were all reclassified as patrol frigates (PF).