I'd have to go with the XBLR-2, better known as the XB-19. I've got a thing for USAAC birds in general, and the B-19 was such a massive machine as to be awe inspiring in that regard alone. There are bigger aircraft today, but none can match the majestic air surrounding the B-19.
As a second choice, I'd go with the XBLR-1, also known as the XB-15.
Here's a link to a page on the USAF Museum site that deals with bombers. Scroll down until you get to the XB-15 and/or the XB-19 for some general info on both.
http://www.wpafb.af.mil/museum/research/bombers/bomber2.htm
Fade to Black...