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

[GeeRam]

Please forgive my ignorance but why is it impossible for them to reproduce the tapered spar tubes?

The tooling no longer exists to do it, so the expense of building new bespoke tooling to draw the tubes with an internal taper, for the small run of tubes needed, is just cost prohibitive. Some of the restoration shops that have worked on B-17's in the USA started looking at this issue several decades ago, and they eventually found that the tooling may have been sold off to China back in the 70's, but the trail basically ran cold after that. Back in the 90's when there were double the number of B-17's still airworthy, it was cost prohibitive to make new tooling, and now with less aircraft, its very much cost prohibitive.