Missions where i can SHOOT down aircraft, WWII or Modern day is fine.
Don't think there are any; best you are going to get is a simulated weapons release.
There are a whole lot of physics involved with combat that aren't present in regular flight; FSX isn't programmed for it.
There is one:
It won't actually shoot down the aircraft, but it will lock on and fire actual missiles at Ai and Multiplayer aircraft. Once the Attack Pack comes out (VRS Product) you will be able to equip any combat aircraft with missiles.... and this time, you will be able to shoot down aircrafts.
Take a look at the screenshots in the FSX Screenshots called, "I Bought a Virus". It shows the VRS Superbug in action.