The Rolls-Royce Merlin powered the Battle of Britain aircraft, the Spitfire and Hurricane, as they defended the shores of Britain against the Luftwaffe in summer 1940. It also powered the Lancasters and Halifaxes of Bomber Command as they went on their missions of destruction to the German heartland. For Americans, the Merlin was the power plant of the Mustang escort fighter that protected the US Air Force B-17 Flying Fortress and B-29 Super Fortress bombers on their daylight raids to the enemy. This book tells the human story of the development of the engine and its operational use during the war, featuring the voices of air crew who relied on this technological tour de force.