B-17 segment... the lost script that became Alien
I can't find this info elsewhere, maybe this should be a Trivia addition... maybe this is only interesting to me 'cause I'm an Alien fanatic ;)
I never watched Heavy Metal, but watching Nostalgia Critic's review I immediately noticed something very familiar in the B-17 segment. I searched IMDB to see if Dan O'Bannon was involved in some way, and here he was, listed as writer for that segment. It didn't surprise me one bit.
O'Bannon's masterpiece Alien was based on an early script he wrote about a B-17 goin' on a bombing mission in World War 2. Suddenly (if I remember well, when the plane is struck a lightning) a Gremlin jumps on the plane and starts making his way through the giant plane, killing the crew one by one. The pilots try to fight the monster and make barricades so that he doesn't reach the cockpit.
Of course the B-17 in Heavy metal has zombies and it's very, very short but you can clearly see the origin of that story - and, as Doug Walker said, it's got some great potential. Too bad they never made a movie about it!