The actual offensive was preceded by three trial phases: Operations gondola (4–16 February 1943), enclose i (20–28 March 1943), and enclose ii (5–13 April 1943). Beset by difficulties, such as the withdrawal of the U.S. Army Air Forces' B-24 Liberator bombers, slow delivery of the ASV Mk. III radar, and lack of aircraft, the operations were nonetheless a success in that they demonstrated an increased efficiency in aircraft allocation and in U-boat sightings.
Air Marshal Sir John C. Slessor, head of Coastal Command, decided to launch the full-scale offensive (Operation derange) on 13 April with 131 aircraft. The repeated, accurate night attacks by the Vickers Wellington medium bombers of Number 172 Squadron, then the only Coastal Command aircraft equipped with new ASV Mk. III radars and Leigh Lights, produced instant although unforeseen results. The failure of the German threat receivers to warn the U-boats of the incoming aircraft and the success of two U-boats in shooting down the attacking planes convinced the German U-boat command that the remedy was to give up the night surface transit and to order the U-boats to fight it out with aircraft on the surface during daylight hours.
Coastal Command aircraft wreaked havoc among the grossly overmatched U-boats during those daylight battles. In May alone, six U-boats were destroyed and seven so severely damaged that they had to return to their bases. In turn, the U-boats accounted for only 5 of 21 aircraft lost by the Coastal Command in the Bay of Biscay that month.
The German withdrawal from the North Atlantic convoy routes following the "Black May" of 1943 allowed Slessor to step up the operation with additional air assets. The Germans took to sending the U-boats in groups in order to provide better antiaircraft defense, yet in June, 4 U-boats were lost and 6 others severely damaged. derange peaked in July, when Allied aircraft claimed 16 U-boats—among them 3 valuable Type XIV U-tankers—compelling Grossadmiral (grand admiral) Karl Dönitz to call off a planned operation in the western Atlantic.
German losses in the Bay of Biscay dropped considerably thereafter, but the air patrols remained a formidable obstacle throughout the remainder of the war by forcing the U-boats to remain under water for most of the time during transit. Although the Battle of the Atlantic was ultimately won around the convoys, the Bay of Biscay Offensive contributed to the success by preventing many U-boats from reaching their operational areas in time to saturate convoy defenses as they had done in March 1943.
Blair, Clay. Hitler's U-Boat War. Vol. 2, The Hunted, 1942–1945. New York: Random House 1998.; Gannon, Michael. Black May. New York: Harper Collins, 1998.; Morison, Samuel Eliot. History of United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal, August 1942–February 1943. Boston: Little, Brown, 1949.; Roskill, Stephen W. The War at Sea, 1939–1945. Vol. 2 and vol. 3, pt. 1. London: Her Majesty's Stationery Office, 1957 and 1960.