Kent and Burton were themselves Americans, which is probably why they erred in one point in their work: bluebirds are not native to England. Also, over the years, it has been suggested that Kent borrowed some of his tune from "Somewhere Over The Rainbow" but though there are some similarities between the two melodies, this seems like a bit of a stretch.
"I'll never forget the people I met braving those angry skies. I remember well as the shadows fell, the light of hope in their eyes. And tho' I'm far away, I can still hear them say, "Thumbs up!" for when the sun comes up... (chorus) There'll be bluebirds over... "
The "angry skies" are obviously where the dogfights were taking place, and "Thumbs up!" was the signal given to the pilots as permission to go. It has also been speculated that the "bluebirds" in the chorus actually refer to the fighter pilots, as the uniforms of both the Royal Air Force and the Royal Canadian Air Force were blue.
The rest of the chorus looks forward to the end of war, when the soldiers will be able to return home and life in a victorious England which will once again be safe, peaceful, and happy.