I don't think its the load, I think its about stress on wing caused by wind forces

B-17 has big, wide and tall wings, which sure will break off if you'll try to point nose up in B-17 too hard...
For using autopilot, I suggest you manually first enter into climb so that your speed drops nearby that speed what you've set it to climb when you turn on auto.
I've had no problem flying B-17 whatsoever...
It can still do fine in 90 degree banking

(real buffers wouldn't want to do that, or they would fly rest of the trip in a chute)