At 18 years old, it was her first film. Her screen test was the unforgettable "You know how to whistle, don't you?" That wasn't intended to go into the film but, well, the director knew when he had a winner.
There is a persistent rumor that an even younger Andy Williams dubbed the singing for this song. It isn't true, but Bacall's very low register was (and is) quite unusual. It really is her singing with the great Hoagy Carmichael here.