All of the series letters beyond D assigned to the B-29 were for
modified earlier versions.
XB-29E was the designation assigned to a B-29 that was converted for
fire-control system testing.
Sources:
-
Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks
International, 1993.
-
The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile,
Doubleday, 1969.
-
B-29 Superfortress, John Pimlott, Gallery Books, 1980.
-
Famous Bombers of the Second World War, William Green, Doubleday,
1960.
-
Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press,
1989.
-
United States Military Aircraft Since 1909, Gordon Swanborough and
Peter M. Bowers, Smithsonian Institution Press, 1989.
-
Jane's Fighting Aircraft of World War II, Bill Gunston, Military
Press, 1989.
-
American Combat Planes, Third Enlarged Edition, Ray Wagner,
Doubleday, 1982.