"Any airplanes ever built with steel wing spars? Fuselages, etc. yes (gas welded but that's another subject) but I don't think any wing spars that were successful."

Recomended book: No HWY by Nevil Shute.

I'm pretty sure tons were, welded tubing as you say. I don't actually know what is in the spar of a Boeing, I am ebarassed to say. Al is certainly in the skin etc... don't know about the caps on the spar. Presumably today - carbon.