Olds,
Perhaps you're misreading the info? The B-17 Pilot Manual I have shows that with the R-1820-97 engine in the B-17F and G you can maintain 46 inches of MP all the way up to 27,000 where you become limited by turbocharger turbine RPM (can't exceed 22,400). You can maintain 41.5 inches MP up to 30,000 and 35.2 inches MP up to 35000.