12/31/2005. Remarks by Bernhard Klein: "SK stands for Skoristnii Kryilo = high-speed wing. This was a Soviet test aircraft. Only one was built to test a flush cockpit design. The cockpit was completely ensconced within fuselage. The front window flipped up and the seat was raised to give the pilot a better forward view during take off and landing. First flight January 1940 piloted by test pilot G. M. Shiyanov."