The Bereznyak-Isayez BI, first flight 10 Sept. 1940. While never used operationally it was always flown with 2 ShVAK 20mm cannon with 45 rounds each. It was found that at speeds over 900km/h the BI would develop an uncontrollable nose down pitching moment, which could not be held by the pilot, which killed Capt. G YA Bakhshivandzhi. This terminated the delayed plan to build 50 production series aircraft as well as waning interest. A version with slightly longer flight duration using two ramjets on the wingtips was tested in the windtunnel.
It handled beautifully but was ultimately killed by the time it took to overcome its problems, and the impracticably short flight times. It never saw combat use (as is known).
(I know some other stuff to, such as it used 50 degrees of flap for landing
)
Oh, the aircraft pictured is the slightly updated version they made after the accident with circular winglets on the elevator and other small changes. This version saw high speed flutter of the tailplane (no accidents). The next improvements included wire bracing the tail and structural changes.