Who developed the amphibious landing doctrine used by the Army on Dday
While it is true that the credit cannot be given to one person, one man who did play a very important role in creating the doctrine that guided much of US Amphibious Warfare during World War II was Earl Hancock Ellis. He developed much of the plans during WWI in the Pacific, but several of the important ideas were used in planning and assembling the invasion in Normandy for D-Day as well.
Some of the important ideas were the idea that the fleet needed to be assembled in port or far from the invasion point and proceed to the landing en-masse. Another important idea was that a great deal of the casualties would come in the sea-shore transition so a great deal of effort should be concentrated on making those plans specific and effective.
The planner for the actual invasion itself was Bertram Ramsay who oversaw the development of Operation Neptune.
If you mean by this question who actually developed the strategy that was followed on D-Day, no one particular person can receive this credit as it was developed by various high-ranking leaders of the Allied forces in many, many planning sessions.
If you mean by this question who was ultimately responsible for its execution and leadership, that would be the American General (and future president) Dwight D. Eisenhower.