Its not automatic. If bomber 1 is fatally damaged, jump to the pilots seat by pressing "1" and then press enter 3 times. (Just like bailing out.
You will now be in Bomber #2. (which stinks) beacuse your fire has been interupted, and the bog is probably having his way with your other 2 planes.
It really is unfair. You should be able to press enter 3 times, and jump to the same place you were in bomber 1. In other words, your in the tail gun of A/C 1. Your hit. You hit Enter 3x, and you should be in the tail gun of A/C #2.
But that's not the case right now.
Also, you can switch to different plane positions by pressing CTRL-1, CTRL-2 CTRL-3. This will put you in the aircraft number you selected. Mind you, you can only gun from these planes, but I think you can turn using your rudder. You do not have to lose a plane to do this. You can hop to other gunner pos. in other planes by simply using the above method.
I hope this helps.