On mission 27, "Aces", you are able to complete the level without having to traverse the entire tunnel. Here is what you do:
Play the level as normal until the tunnel is opened. Instead of flying into the entrance, fly North towards the exit of the tunnel. It should be directly North of the entrance.
Once you find it, approach it from the side so you can just barely enter the tunnel and turn back out. If done correctly, Bartlett will say"I'm right in front of you Kid. On the count of three, break right."
Once he says that, fly back to the entrance of the tunnel, and enter it. Fly through it as you normally would, but when you destroy the target inside, the cut scene of the Razgriz exiting the tunnel will start, and the level will end.
Note: You can not get the special colored aircraft inside the tunnel if you do this.