The Longest Day, the epic 1962 movie about D-Day, was released about two years before Dell's Combat #11 (January-March 1964). So, in the days before movies were expected to earn their profits more gradually and stuck around the theaters longer, the decision to do an issue highlighting the Normandy invasion might be related to that.
Also, the comic book hits pretty much the same highlights the movie (and the Cornelius Ryan book upon which the movie is based) hits. But, of course, a lot of those highlights are the historically important ones, so that's not so odd by itself. Also, 1964 was the 20th anniversary of the invasion, which also might have been a factor.
Whether Dell was inspired by the movie, the comic itself is a really good one. It has wonderful Sam Glanzman art throughout and, though we are given a couple of fictional point-of-view characters, does an effective job of giving us an historically accurate overview of the invasion.
We see, for instance, that the paratroopers who landed in the early morning hours were scattered all over the place, forcing them to scrounge for equipment and slap together ad hoc units to accomplish their missions.
the capture of an important bridge by British commandos. It's this, actually, that makes me think that the movie inspired the idea for the comic. In real life, the British discovered--after capturing the bridge--that the Germans had never planted demolition charges on it. In the movie, the producers opted to add drama to the battle by showing the British disarm the charges in the nick of time.
The comic also implies the bridge actually is rigged to blow--though again, to be fair, this is in the form of an officer telling his men to check for explosives and does not show them actually finding them. But the comic doesn't make it clear that the explosives were never attached to the bridge.
Several pages concentrate on Omaha Beach, where American troops ran into a meat-grinder, but eventually managed to fight their way inland. Like the movie, though, the comic does give time as well to the efforts of British, Canadian and French troops.
The comic is in the public domain now and can be read HERE.