Completed in March 1943 as USS Block Island (AVG 21), on 20 August 1942 reclassified (ACV 21) and on 15 July 1943 finally (CVE 21).

USS Block Island (CVE 21) made two trips to UK with cargoes of US Army fighters and four anti-submarine cruises as part of the hunter-killer group TG 21.11, this group sank six u-boats (U-220 and U-1059 by her aircraft, U-801 and U-66 shared with escort vessels and U-709 and U-603 by the escorts alone). She received two battle stars for her WWII service. 

On 29 May 1944 U-549 slipped undetected through the screen of the hunter-killer group TG 21.11, formed around USS Block Island (CVE 21) (Capt Francis Massie Hughes, USN) and at 20.13 hours fired three T-3 torpedoes at the carrier about 300 miles west-northwest of the Canary Islands. Two of them struck the ship, followed by another torpedo as coup de grâce about eight minutes later that caused the carrier to sink at 21.55 hours.

At 20.40 hours USS Barr (DE 576) (LtCdr H.H. Love, USNR) was damaged by a Gnat in the stern and a second Gnat missed USS Eugene E. Elmore (DE 686). But the U-boat was then attacked and sunk by the latter destroyer escort and USS Ahrens (DE 575), which then picked up 674 survivors from the carrier. 277 other survivors were rescued by USS Robert I. Paine (DE 578). They were all landed at Casablanca on 1 June.

Six Wildcat fighters from VC-55 were in the air when USS Block Island was torpedoed and this left them with no place to land. They headed for the Canary Islands, but all aircraft had to ditch at night after running out of fuel and only two of the six pilots were rescued.

More information about the engagement between the hunter-killer Task Group 21.11 and U-549, including photos of the sinking escort carrier and the damage report can be found in the U-boat Archive: U-549 sunk by USS Eugene E. Elmore and USS Ahrens on 29 May 1944