To minimize crew fatigue it is important that they have room to move, relax, or even nap. The B-36 was big enough (and design for missions long enough) that it could carry a second crew and feature a berthing area and galley.
What's the WWII plane that has the endurance of 18 hours? Even then the B-36 isn't modern, it isn't modern at all. The B-36 would have still required a lot of concentration, that's not to say the planes nowadays don't.
The B-36 was almost a World War 2 design anyway.
The B-36 program actually began in 1941 and made its first flight Aug. of 1946. As far as the technology involved it was for all intents and purpose a WWII desgin.
That's maximum endurance 18 hours, in that case the pilot would rest while the co-pilot would fly. If there was any problems he'd call in the resting one...