By January 1943, the Allies had developed technology and tactics that could take on the German U-boat threat. But an area of the mid-Atlantic, out of the range of Allied aircraft, was being targeted by the U-boats hunting convoys. So America introduced the long-range Liberator bomber in an attempt to close the air gap and win the war in the Atlantic. And Germany sent U-boats into the English Channel, hoping they could play a crucial role in preventing the D-Day landings.