How long did it take to plan the landing at Normandy?

It took at least several years to plan out all of the various details of Operation Overlord, the largest amphibious landing in world history. Over 160,000 Allied soldiers were committed to hit the beaches that day; by the end of August, over three million troops would be in France.

In 1942, British Prime Minister Winston Churchill unofficially referenced a lack of resources necessary to succeed with such a large undertaking. There was a proposal for an autumn of 1942 seizure of French ports, called Operation Sledgehammer. There was also Operation Roundup, a plan put forth for an invasion of northern France in the spring of 1943, which was delayed, expanded, and changed into Operation Overlord, utilizing aspects of Operation Sledgehammer and Operation Roundhammer.