... before the practical development of in-flight refueling, if you wanted to go a really long way, you had to have a really big airplane. Lots and lots of fuel meant big fuel tanks, bigger wings for more lift, bigger tails for better steering in the high-altitudes which meant bigger engines etc. etc. There were a lot of designs. And a lot of wacky ideas.
Remember- in the late 1940's and early 50's there was a distinct lag between long range radar coverage, interceptor technology and the chance that this big baby (or a B-36) just might be able to slip through to it's target!