Here you can find all AI/Hidden planes, they are 17+1 in total.

AI Escort Bomber

AI Escort Enemy

AI Opponent

AI Opponent Fast

AI Opponent Glider

AI Opponent Medium

AI Opponent Off Road

AI Opponent Race

AI Opponent Race Car

AI Opponent Slow

AI Opponent WW2

AI Refuel Tanker

Designer Tutorial 1

Designer Tutorial 2

Drone

P-51 WW2

To get 18th plane, just press new aircraft button :-)