What a beast.
Here’s a drawing showing the progression of piston-powered heavy bombers:
Top to bottom:
Martin GMB MB-2
Click for better look.
Help Murdoc out on the second one.
UPDATE: Nicholas correctly points out that the second plane is a Martin GMB MB-2, one I hadn’t even considered. (To tell the truth, one I hadn’t even heard of, I think…) Well done.
Also, another reader notes that the Pima Air & Space Museum in Arizona is in the process of restoring a B-36. See the page for lots and lots and lots of great photos.