A wrote a program that does it,

https://dochub.com/yragroneelf/b9nygm/amaj-7ths

I can translate to any key and have good voice leading.
I don't generate MIDI, just figure out where to put dots on a grid.

If you turned in what band in box generates in a music theory 101 class you would get a F. Its 'all' parallel voice leading. (Maybe not all but at first look I think it may be)

Korg Karma will do it to some degree, if you voice lead in the input the output will match. although it does not have the wonderful layout that band in box has where you can map out the whole song ahead of time. Biab can place the fill in right spot and so forth.

In order to generate bass lines in is has to figure out the root, 3rd, 5th it has to do some analysis.