I'm sure the top speed was with the outboard turbojets burning. A 4x turboprop version of the B-47 sure would've been interesting and probably would've had certainly longer range, but likely would've cost it some significant speed. A mixed-powerplant version just like the D presents an issue in maintenance and cost with two different powerplants.
The only thing I'd love to see more than this is the XB-47D Turboprop Test-bed. Made from two ex- B-47Bs (Tail Nos. 51-2046 and 51-2103), these two aircraft retplaced the inboard J47-GE-23 jet engine pairs with a single Curtiss-Wright YT49-W-1 turboprop of 9710shp. Top recorded speed was 597 mph at 13,500 feet, the fastest yet achieved in level flight by a propeller-driven aircraft (even faster than the 575mph TU95/142 Bear). If anyone out there has any video on the XB-47D, PLEASE post it!
