Why was D-Day successful?

Film clip from "Special Invasion", a film produced by War Pictorial News showing actual footage of aerial attacks on Hitler's West Wall coastal defences before and after D-Day

Crown copyright

1944 Tuesday June 6th, the invasion of Western Europe, a date with history! From hundreds of airfields on Britain, thousands of aircraft roared out across the Channel in a shuttle service of destruction. Theirs, a vital task: the softening up of the German coastal defences, Hitler's formidable West Wall. The bombing and strafing [attacking using low-flying airplanes with machine-gun fire] of communication channels carrying ammunition and weapons from inside Germany to still further strengthen those same coastal defences. Theirs too, the job of keeping the Luftwaffe out of the air