To unlock the x-o2 stealth, get a rank of "s" on all 18 
missions under the normal mode. In order to get the second 
x-02, finish the game getting an "s" rank under the hard 
difficulty setting. This will unlock a black plane. 
Finish the game under the expert mode and this will unlock 
the last x-02, this one is silver but once again you are 
required to recieve an "s" on all missions.