Can anyone explain what the Bm7 is doing in Django's intro other than sounding really cool?
As for the rest here's how I hear it:
First he paraphrases the melody on top of a Bb triad then moves to a kind of rootless voicing of the Bbm and then to the Fmaj7 (all in keeping with the chord progression so far). Then the inexplicable Bm7. Bbdim resolves to F, and an F7#5 sets up the Bb6 which begins the chord progression of the song.
Really love this intro. I wonder if Django would just ad lib these things or if they were premeditated at all. In a Sentimental Mood is another of my favorites.