The short answer is that they did, but it didn't always work.

The slightly longer answer is that the Allied bombing campaign before D-Day was concentrated on logistical and strategic targets: road and railway junctions, stations, ports and harbours, marshalling yards and so on. The idea was to isolate the beachheads and make it difficult for the Germans to bring up supplies and reinforcements. Another reason for this was to mask the fact that the target was Normandy.

On D-Day itself the target was the fortifications and entrenchments, but in the dark, with the cloudy weather, results were patchy. The defences at Utah beach were hit hard, but further along the coast at Omaha, bombers were unable to hit the trenches (a tiny target from the air, don't forget) and defences there remained largely intact.

Source: Own knowledge, with http://www.dday-overlord.com/eng/dday_bombardments.htm as an aide-memoire.