To get the incredible XFA-27 you must first complete the game on the 
normal level. Then play on the difficult level until you reach the low 
altitude attack mission. A special squadron over the south of the target 
called Fox Force Four. This squadron is made up of four YF-23A's . Attack 
and destroy all four and on the debriefing screen you will have a new 
medal and the new aircraft available message will appear.