FLY OR DIE MULTIPLAYER is a better version of this type of game. It has different planes, lot's of competition... it's always fun regarless of your level of play... There you can team up with other players and coordinate differente attack strategies.
Finished in 17 days!
It can be beaten in 16 though.. as I came pretty close to it.. and made some mistakes in the beggining.
it is unlikely that it can be beaten in 15 days.
 "Steambirds: Survival"){kind=icon}
 "SteamBirds"){kind=icon}
 "Starfighter: Disputed Galaxy"){kind=icon}
 "Learn to Fly"){kind=icon}