The sign accompanying the B-17 at the Barksdale Global Power Museum (formerly the 8th Air Force Museum) makes reference to the "Executive Fortress" program.

I'd never heard of this program before, and a fair amount of Internet searching failed to provide much additional information (other than that the program was apparently actually called the "Fortress Executive" program). The only diagram I could find when initially researching it was on a forum, low-resolution and photocopied from a book, with the accompanying binding-induced curvature.

As I was updating dead links on the B-17 page, I found that the forum on which I originally found the diagram now requires registration, so I fetched and reconstructed the diagram, fixing the curvature, and performing general clean-up on it.

From a Fedora Lounge forum post.
Reconstruction by heroicrelics.

The caption reads

In a move to generate some sales to civilian business, Boeing circulated this drawing of their postwar "Fortress Executive", advertising the availability of surplus B-17s for peacetime conversion. On paper the idea looked good, but the B-17 was just too big a machine for a company aircraft and not big enough for commercial airlines. (Boeing).

If you happen to know anything more about this program, or have a better scan of this diagram, please drop me a line.