I'm pretty sure Christopher Burke's Renner biography -- The Art of Typography -- contains documentation of Futura's development, including discarded designs and early versions. I don't have my copy handy to double-check but it's a safe place for you to start.

"Sketch"

This thread a while back linked to a thesis on Futura, which, if I remember correctly, goes into the form and Renner's process quite a bit. I've been meaning to finish reading it; maybe now I'll get around to it.

The Burke book doesn't feature anything that is not also in the Charles thesis. You might try contacting Burke to see if more drawings exist than those he reprinted; his research included a lot of digging through the old Bauer files that turned up an assortment of long-forgotten goodies.