The fourth volume in this continuing series relates the growing strength of the Allied air forces, particularly the close partnership between the Royal Air Force fighter forces and the American 8th and 9th Air Forces. For a time, the RAF took responsibility for escorting American bombers into enemy-held territory and also 'educated' the newly arrived Thunderbolt and Lightning units into the dangers of air combat in Europe. Towards the end of 1943, two important things happened. The first was the discovery of flying-bomb launch sites on the enemy coastline, the second was the arrival of the superb North American P-51B Mustang. Possessed of enormous range, and with an impressive performance, the Mustang gave the USAAF the means to strike at the German heartland and to force much of the Channel-based Luftwaffe fighter force back to defend the Reich. The period from the summer of 1943 until spring 1944 would prove to be decisive in securing for the Allies the conditions necessary to mount and carry through a successful invasion. The vital factor was that of not just air superiority, but air supremacy without which the enormous naval resources required to transport ground forces across the Channel would have been at great risk. This volume concludes with Operation Overlord and shows not merely the massive air supremacy achieved over Normandy, but also the vital role played by close air support as the beachheads were consolidated and the ground forces began their advance towards Germany.