But you don't need to adjust the trim on the P-38 to do it.
ack-ack
You are, in a sense, adjusting the trim when you drop flaps. Trimming for level flight without flaps is a nose up trim with flaps. My point is that every fighter will loop itself with no pitch input until you run out of fuel, with or without flaps, but you have to correct for the torque except in the P-38. It's a nice lesson in not pulling too hard.