And here you thought I'd start with the Nazis.
The I-153 Chaika was relatively state of the art for a biplane. It featured retractable landing gears, and could mount external munitions (rockets and bombs) to compliment the guns it was built with. Some were even flown against Luftwaffe fighters early in the Great Patriotic War.
At one point, the Soviets did flight tests of an I-153 Chaika that had a ramjet engine mounted under each wing. A goddamn jet powered biplane.
A series of test flights showed a 50km/h increase in average speed, making the plane still pathetically slow compared to modern German and Soviet fighters like the Yak-1 and Bf-109.
When the Nazis had experimental weapons, it was the stuff of Science Fiction. When the Soviets had experimental weapons, it was the stuff of Slapstick Comedy.