FAA grounds all B-17E,F, & G models

[GeeRam]

A tapered spar tube?

There are several ways to produce a tapered tube, on any scale.

If the tube, be it steel or of some interesting aluminum alloy, is ductile enough, it cam be run into a gigantic "sizing" die, or "spun" with traveling dies", squeezing the parallel tube into a "conical" tube. This process will ado elongate the tube, in much the same way the starting blanks for hammer-forged barrels "stretch" as they are hammered.

NOT ALL ALLOYS are equal. Choose wisely and get opinions from several actual metallurgists.

IF the spar is readily removed and replaced, IFF replacements are available, then do it.

Failing that; perpetual hangar queens. Wreckage-strewn craters are a "different" sort of "interesting".

The tubes are constant OD, with an internal taper reducing the ID over part of the length, which is why 20 years of investigation has drawn a blank to getting them remade in the correct material that is commercially viable for the amount required.
There has been a lot of investigation done on this by the various B-17 restoration specialists, and they've drawn a blank.