KP-5 ring sight offset to starboard. Some aircraft had bomb racks to carry two bombs. The first prototype was completed in early 1928 and made its first flight on 21 February. Its manufacturer's trials were finished by 10 March and the state acceptance trials by 14 April. The pilots of the NII VVS ( – Air Force Scientific Test Institute) criticized the lack of directional stability at high speeds and a slight problem in control response between maneuvers. The area of the vertical tail was increased and the elevators were given horn balances to alleviate the first problem while split ailerons addressed the second problem. Since production had begun before the aircraft was actually approved for service use, the first forty aircraft were completed with the smaller tailplane. A second prototype was completed in August 1928 and tested a different propeller optimized for high speed which increased the top speed to , although it lengthened the take-off run. The first 39 aircraft completed, plus the two prototypes, used imported engines, but the remainder used the license-built Mikulin M-17. Approximately 400 were built, with Gordon and Dexter citing sources that state 389 or 399. They also provide a yearly production table that lists 35 built in 1928, 47 in 1929, 250 in 1930 and 55 in 1931, which add up to 389 when the two prototypes are included. DI-2 The DI-2 ( Dvukhmesnyy Istrebitel - two-seat fighter) was an enlarged two-seat variant. It had an extra frame added to the fuselage, an extended wingspan and an enlarged rudder. Two 7.62 mm DA machine guns were mounted on a Scarff ring in the observer's cockpit. A prototype was completed in early 1929 and made its first flight in May of that year. However the prototype crashed due to stabilizer failure in a dive later in 1929, killing the pilot. Operational use Initial deliveries in 1929 were to units in the Belorussian Military District where they replaced the Grigorovich I-2. They equipped the 4th and 7th Squadrons (–Air Squadron), later the 106th and 107th Fighter Squadrons (—Fighter Air Squadrons) at Smolensk; the 13th and 5th Squadrons, later the 108th and 7th Fighter Squadrons, at Bryansk, the 9th Squadron, and the 17th and 19th Squadrons, which later became the 116th and 117th Fighter Squadrons. Units based in the Ukraine began to receive theirs the following year. They equipped the 3rd Squadron, later the 109th