As the movie was starting with people dancing, swing in particular, I had to think of some music that really fed the era.
When I was thinking about 40's music I started to look at swing music like: BOOGIE WOOGIE BUGLE BOY OF COMPANY B by the Andrews Sisters
But I thought that it was too fast to dance properly to.
So I started to think logically about the mood between the characters and the story. I remembered this song by Vera Lynn that I had listened to when I was studying WW2 in GCSE History. Bluebirds Over The White Cliffs Of Dover.
This was perfect as it was slow enough to dance to and it was 3 minutes long. And it was significant to the era of the movie. Because the bluebirds were the British spitfires flying home after WW2.