I backed up to them in a P-38.
Get a very, VERY slight amount of forward movement to unlock the brakes and then fire your guns. The "thrust", e.g. kick, from the guns will start you rolling backwards. Once you are where you want to be, stop the plane.
I recommend using the throttle to stop the plane because, at least in the P-38s case, it is not balanced to stop in reverse and will tip backwards.
No, you may not ask how I know that.

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We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother
Sisu
-Karnak