The 747 used to have a selectable Gear as you can see from this photo:
http://www.!!!!!!!!!!!!!!/open.file/349788/L/
I believe it was a form of speed brake possibly for an emergency decent, it was later found to be redundant so disabled. I saw the modification on a Polar Air Cargo some years ago so a mate and I followed it up and discoverd the wing gear used to be able to be selected down independantly.