By D-Day, June 6, 1944, the Allies had been planning for the invasion of Europe for more than two years. In August 1943, the Combined Chiefs of Staff had approved the general tactical plan for the invasion, dubbed Overlord. General Dwight D. Eisenhower, Commander of the European theater since February 1944, would be responsible for carrying off this bold gambit. The Allies' main strategy, in Elsenhower's words, was to

. . . land amphibious and airborne forces on the Normandy coast between Le Havre and the Cotentin Peninsula and, with the successful establishment of a beachhead with adequate ports, to drive along the lines of the Loire and the Seine rivers into the heart of France, destroying the German strength and freeing France.

The Allies believed that the enemy would resist strongly on the line of the Seine and later on the Somme, but surprisingly, once ground forces had broken through the relatively static lines of the bridgehead at Saint-Lo and inflicted heavy casualties on enemy troops in the Falaise Pocket, Nazi resistance in France disappeared. British and American armies swept east and north in an unimpeded advance which brought them to the German frontier and the defenses of the Siegfried Line.

Air Power: Critical to Success on D-Day

From the beginning Eisenhower and the rest of the combined forces planners recognized that air power would be critical to success of Overlord. Experience had taught planners to avoid facing hostile air power over the battlefront. This meant that the Luftwaffe would have to be destroyed, but not at the price of sacrificing vitally needed air support missions for air superiority ones.

Fortunately, in early 1944 the Luftwaffe was on the skids. By the fall of 1943, Republic P-47 Thunderbolts equipped with long-range 'drop' tanks were inflicting heavy losses on German fighters over Occupied Europe and in the German periphery. Then, in December 1943, the North American P-51B Mustang entered service. Featuring superlative handling qualities and aerodynamic design, and powered by a Packard-built Rolls-Royce Merlin engine, the P-51B (and its successors, the P-51C and P-51D) could escort bomber strikes to Berlin and back, thanks in part to a symmetrical wing