One of my Grandpas flew B-24s in the Pacific and will talk your ear off about it. The other parachuted into Normandy before D-day and didn't like to talk about it much, but there was one thing he said that always stuck with me. He said no one knew exactly when the invasion was going to happen, until one day when they looked up and "the sky was black with planes". Thousands of planes in the sky at once. The scale of some of those missions is completely unfathomable today.