The initial plans were constrained by the number of landing craft, most of which were already committed in the Mediterranean and in the Pacific. In part because of lessons learned in the Dieppe Raid of 19 August 1942, the Allies decided not to directly assault a heavily defended French seaport in their first landing. The failure at Dieppe also highlighted the need for adequate artillery and air support, particularly close air support, and specialised ships able to travel extremely close to shore. The short operating range of British aircraft such as the Supermarine Spitfire and Hawker Typhoon greatly limited the number of potential landing-sites, as comprehensive air support depended upon having planes overhead for as long as possible. Morgan considered four sites for the landings: Brittany, the Cotentin Peninsula, Normandy, and the Pas-de-Calais. As Brittany and Cotentin are peninsulas, the Germans could have cut off the Allied advance at a relatively narrow isthmus, so these sites were rejected.

The Pas-de-Calais, the closest point in continental Europe to Britain, was the location of launch sites for V-1 and V-2 rockets, then still under development. The Germans regarded it as the most likely initial landing zone and accordingly made it the most heavily fortified region; however, it offered the Allies few opportunities for expansion as the area is bounded by numerous rivers and canals. On the other hand, landings on a broad front in Normandy would permit simultaneous threats against the port of Cherbourg, coastal ports further west in Brittany, and an overland attack towards Paris and eventually into Germany. The Allies therefore chose Normandy as the landing site. The most serious drawback of the Normandy coast – the lack of port facilities – would be overcome through the development and deployment of artificial harbours.

The COSSAC staff planned to begin the invasion on 1 May 1944. The initial draft of the plan was accepted at the Quebec Conference in August 1943. General Dwight D. Eisenhower was appointed commander of Supreme Headquarters Allied Expeditionary Force (SHAEF). General Bernard Montgomery was named commander of the 21st Army Group, which comprised all of the land forces involved in the invasion. On 31 December 1943, Eisenhower and Montgomery first saw the COSSAC plan, which proposed amphibious landings by three divisions, with two more divisions in support. The two generals immediately insisted on expanding the scale of the initial invasion to five divisions, with airborne descents by three additional divisions, to allow operations on a wider front and to speed up the capture of the port at Cherbourg. This significant expansion required the acquisition of additional landing craft, which caused the invasion to be delayed by a month until June 1944. Eventually the Allies committed 39 divisions to the Battle of Normandy: 22 American, 12 British, 3 Canadian, 1 Polish, and 1 French, totalling over a million troops.