Further development was halted in 1939 and the I-21 designation was re-used for the Pashinin I-21.
(The development of the MP-6 gun can be traced back to 1937. The initial version was tested in the spring of 1940 on a Bf 110 bought from Germany, because there was no suitable Soviet aircraft on which to mount it. In the summer of 1940 it was tested on the Pashinin I-21.)