The period between WWI and WWII represented a period of rapid growth for aviation. Planes shed their top wing and started to get more streamlined and faster. Unfortunately, I didn't capture too much of that in my trip down to the Air Force Museum. But here are some examples.
This is the P-26 Peashooter. It represents the first all metal aircraft used by the US and one of the earliest monoplanes. This was built by Boeing.
This is the North American A-17. It never really saw much service.