The Normandy region in general, and most of Northern France had been receiving extra attention from Allied bombers of all sizes and types for weeks in the lead up to the invasion. From staging points, railway centers, depots and command points, and even just units caught out in daylight were hit. In the name of attriting the enemy reserves, and hampering their ability to concentrate against a landing that would occur at an unknown time and place.

However the actual prelanding destruction of beach defenses and establishment of a beachhead would rely far more on surprise than say many landings in the Pacific that would see days on days of bombardment by warships and carrier aircraft before a landing.

But that is not to say that no pre invasion bombardment was conducted. The plan was for the fleet to approach under the cover of night to achieve surprise, give minesweepers a chance to work, and begin deploying the slow landing craft in relative safety. Then the last hour or so before the troops hit the beach as light levels began to increase the naval task force would start a pre planned bombardment of enemy strongpoints and beach defenses, followed by a wave of bombers. The naval groups also had plenty of firepower for the job, including 5 battleships and a similar number of heavy cruisers and numerous destroyers who would all also then continue to provide gunfire support as directed by landing teams ashore(and some who would literally beach themselves after getting into pointblank gun duels with German shore batteries). The landings would also theoretically have been able to support themselves too as very early int he landing order the amphibious Duplex Drive equipped Sherman tanks should have made it ashore too, and in some sectors at least they succeeded to material help there.

It was not as effective in some areas as might have been hoped, but still all told the landings went off pretty well on 4/5 beaches, it was only OMAHA that became the stubborn holdup, and had resistance continued follow on forces could have been routed elsewhere, and while more difficult, the remaining defenders could have been encircled and the beachead made hole and larger that way.

You may be interested in this read, its long, but an excellent breakdown of the various planning considerations, assumptions, and contexts behind the plan for NEPTUNE, which was the name for naval component of the landings. https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/o/operation-neptune-invasion-normandy/chapter-4-neptune-operations-plans.html

The navy had been shelling the area for quite some time prior to the landings. Additionally, the defenses were not expected to throw the assault back into the sea from the beach, rather the Allies were more worried about the counterattack from further inland before a foothold could be gained. That’s why Paratroopers played such a large role. While the beach defenses were built to inflict heavy casualties, and did, they were not expected to throw an assault back into the sea by themselves. Shelling and bombing were focused more on what lay further inland, past the landing areas.