After submission of his proposal to TsKB (Tsentrahl'noye konstrooktorskoye byuro - central construction bureau), there was an 18-month delay before permission was given to proceed.

The major component of the Stal-6 was the V-1570 Curtiss Conqueror steam-cooled V-12 engine, this was closely cowled and defined the maximum sectional area of the fuselage, which transitioned smoothly from nose to tail with a flush cockpit canopy.

Flight testing began in the autumn of 1933 piloted by A.B.Yumashyev and P.M.Stefanovskii, revealing exceptional performance and setting a Soviet speed record, but severe problems with the steam cooling system as well as the bad view from the cockpit were insurmountable.

The need for steam cooling was soon to be negated by the widespread availability of ethylene glycol-based coolants, which had a better specific heat and a much higher boiling point.

The Curtiss Conqueror steam-cooled V-12 engine was replaced by an M-100A (licence-built Hispano-Suiza 12YBR), this also was closely cowled and defined the maximum sectional area of the fuselage, which transitioned smoothly from nose to tail except for the cockpit which was enlarged to improve the view of the pilot.