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.
Warbird History--B-29 Superfortress, Chester Marshall, Motorbooks
The Boeing B-29 Superfortress, Mich Mayborn, Aircraft in Profile,
B-29 Superfortress, John Pimlott, Gallery Books, 1980.
Famous Bombers of the Second World War, William Green, Doubleday,
Boeing Aircraft Since 1916, Peter M. Bowers, Naval Institute Press,
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
American Combat Planes, Third Enlarged Edition, Ray Wagner,