I read with interest a book about operating a B-24 during the Late Great Hate. (It was writtten by George McGovern, spectularly failed Presidential candidate, actually. He was a B-24 pilot during WWII when he got shot at too plenty. He lost the election to a guy who sat out the war on a rock in the Pacific playing poker.) There it was set down in black and white that the brainiest candidates were selected for Navigator with the less-bright being chosen as Pilots.
On the other hand, all the ex-Strategic Air Command guys I flew with were uniformly scathing in their opinions of Navigators. Perhaps one could put that down to jealousy.