In 1943 no principle existed within the U.S. Army for assaulting a heavily fortified coastline and the only published advice in an Army Field Manual was that assaulting troops should “avoid such defences and take them from the rear”.
Lt. Col. Paul W Thompson, assigned the mission to produce a doctrine and then train troops to successfully mount an amphibious assault somewhere on the French coast began by convening a month long conference in London in May 1943, seconding military experts from every service to thrash out a workable method of neutralising German beach defences. He called on experimental projects, collected vast amounts of data, photographs and intelligence and drew upon the combat experience of veterans of similar amphibious landings including the raid on Dieppe.
Every aspect of the problem was considered, in particular terrain and topography to be encountered on the Normandy beaches selected for the American assault. This was probably the most crucial element that dictated the whole doctrine for unlike the British and Canadian assaults which would cross sandy beaches onto undulating grassland, the Americans would immediately be faced with steep bluffs and only a few narrow valleys leading off the beach onto the plateau above "Omaha". Or, narrow causeways across inundated meadows and fields that had to be secured before any inland advance could move off the beach at "Utah".
While the Canadians and British could therefore immediately use their tanks, the Americans had to seize their beach exits first, leaving them no alternative but to attack the defences with infantry, landing their tanks once access off the beach had been secured.
So the assault had to be made by specially equipped and trained infantry sections and training therefore had to be based upon several assumptions including pillboxes and gun emplacements neutralised by flat trajectory, high velocity gunfire. Naval firepower was expected to provide this but confidence in this eventuality gradually waned with training experience and alternative methods of delivering this initial barrage were explored and integrated into the American D-Day assault plan. Artillery firing from landing craft as they approached the shore was one idea adopted and practised at the Assault Training Centre.