I actually did that mission a few days ago and it played out exactly like you said. As I gathered you are simply supposed to survive the fighter attacks. For me too, the simplest thing was to fly out of the city, try to keep the ME262's at bay with my rear gunner and waited until enough of them got shot down by my allies.
But yeah, was a little anti-climatic, especially if you fly back and destroy the last 5 defenses in one swoop and you're done! Hero of the World achievement unlocked... a little cheesy for my taste
