PRE-WWII. I think if I were ever to do a non-WWI model it might be one of the "between the war" flyers. The the time WWII rolled around aeronautical engineers had already eliminated most of the "funky" stuff. That and with WWII scale you're also talking about $300+ smackeroos for scale struts.

But more than anything else, I'm not drawn to the standard WWII modeling techniques of SHEET, GLASS, PRIME, and DECORATE. If I did ever do a metal skinned aircraft you can be sure it would have actual metal sheeting.