Battle of Britain 2, you can play the campaign at level of RAF or Luftwaffe Commander, and then jump into the cockpit in any dogfight or flight on the map at will.
Last night I was in the middle of a Luftwaffe campaign - August 20. Put together a raid of 200 bombers and 300 escort then fragged into one of the Bf109 escort squadrons over France and rode shotgun to the target. Amazing being part of something that huge - there were aircraft all the way from Britain to France as far as the eye could see. The RAF put up about 200 fighters to attack us and in no time I was down among the treetops duelling with a Hurricane. Fighters and bombers were falling like burning leaves all over the countryside. I brought the Hurricane down but my engine started smoking. I nursed my Emil up to 2,000 feet over the Channel before the engine cut out, and I had to bail. Made it down into the drink and into my raft.
Just another day at the office...
PS the graphics, weather etc these days are a treat too...
Image courtesy my favorite screenshotter, Stalkervision

Plus it is only $20 USD and has a big group of freeware developers supporting it with patches and enhancements etc. If you want campaign immersion, these guys now have modelled every fighter skin in the entire Battle! I bought my copy here:
http://www.a2asimulations.com/bob/It says US customers only but I bought from UK using Paypal.
It isn't perfect of course. No multiplayer so although the AI is good, you will be able to beat it with practice. I find it is pretty poor at low level, so if I have an enemy on my tail I can't shake, I drag him down into the weeds and he tends to break off below 200 feet. And the Luftwaffe campaign is damn hard, I have never managed to win it (but I'm still trying!)
PS