Atomic coordinates for DNA bases as they appear in a B-form or A-form duplex
I played around with a program called GROMACS about 4 years ago and don't remember much, but I do remember doing a tutorial simulating a DNA double helix and part of it included making the DNA by giving it a sequence and running a script. It can probably make A or B form.

EDIT: This was a long time ago and I forgot. It wasn't GROMACS, it was Amber. Look here http://ambermd.org/tutorials/ Tutorial B1 looks like the same tutorial I did with the DNA.