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The Polikarpov I-16 was the last of Nikolai Polikarpov's fighter designs to enter production, and was the most important fighter aircraft in the Red Air Force by 1940. It was also the first monoplane fighter with an enclosed cockpit and retractable undercarriage to enter front line service anywhere in the world, but a lack of suitable replacement aircraft meant that it remained in production until 1941, by which time it was virtually obsolete.

The first sketches of the new aircraft were produced in 1932, when Polikarpov was working as Sukhoi's deputy at Brigade No.3 of the TsKB (Central Design Bureau). This was originally the cover name for a team of designers who were working while interned, after being charged with a wide range of crimes as part of an attempt by the Secret Police to gain control of the aircraft industry. Polikarpov himself had been charged with sabotage in 1929. The TsKB designation continued to be used long after the designers had been freed. Serious work began in 1933, when Polikarpov was head of Brigade No.2.

Polikarpov wanted to build his new fighter around the 700hp Wright R-1820 Cyclone radial engine which was expected to enter licensed production in the Soviet Union. The negotiations proceeded very slowly, and so alternative engines had to be used for the two prototypes. The first received a 480hp M-22 engine, a Soviet-built version of the Bristol Jupiter VI, while the second was powered by a 600hp Wright Cyclone 1802-F-2 that Polikarpov had acquired.

The prototypes were given the designation TsKB-12 as the twelfth aircraft to be designed by the Central Design Bureau (TsKB). Work on the prototypes began in June 1933, and both were ready for their maiden flights by the end of the year.

The first prototype (M-22 engine) made its maiden flight on 30 December 1933, with skis instead of the wheeled retractable undercarriage. The Cyclone-powered second prototype made its maiden flight in January 1934. The high regard in which Polikarpov was held at this date was demonstrated in November 1933, when the Soviet Council for Labour and Defence decided to put the new aircraft into series production at zavod 21 at Gor'ky. A small number were also to be built at zavod 39, the factory attached to the design bureau.

Both prototypes took part in state trials from 16 to 27 February 1934. The first prototype reached a top speed of 190mph at sea level and 175mph at 16,400ft. The second prototype was faster, with a top speed of 215mph at sea level and 195mph at 16,400ft.

In March the first prototype underwent trials with the retractable wheeled undercarriage. This time its top speed rose to 223mph at sea level and 201mph at 16,400ft. These were good figures for 1934, and suggested that the Cyclone powered I-16 would be on a par with the best foreign fighters of the period. The standard German fighter of 1934 was the 186mph Arado Ar 65, while in Britain the Gloster Gladiator biplane made its maiden flight on 12 September 1934, and had a top speed of 236mph at 10,000ft. When the second I-16 prototype received a Cyclone F-3 engine later in 1934 its top pseed rose to 271mph.

Development was briefly held up when the second prototype was damaged in a crash, but it was rebuilt with a new cowling, the Wright Cyclone F-3 and some other minor changes and tests resumed in September 1934.

The I-16 would soon be left behind by more modern aircraft. In Germany the prototype Bf 109 made its maiden flight in September 1935. The Hawker Hurricane first flew on 6 November 1935, and its prototype reached 315mph, while the Spitfire made its debut on 5 March 1936, and had a top speed of 349mph. Production standard I-16s didn't break 300mph until the introduction of the I-16 tip 24 in 1940, and even then most standard aircraft failed to reach their full potential. The main limit would prove to be the single row radial engines used to power the aircraft. These were soon outclassed, first by the inline engines of the Spitfire or Bf 109, and then by the multi-row radial engines that powered later American fighters.

Early trials revealed that the I-16 was fast and very manoeuvrable for a monoplane, but also somewhat difficult to fly and not suitable for novices. As a result a two-seat trainer, the UTI, was ordered into production for use as a conversion trainer, and new monoplane basic trainers were eventually needed. Over time many of its bad habits were eliminated, and the later versions were considered to be rather easier to fly.

The I-16 had a stocky barrel-shaped but streamlined fuselage, built around a wooden monocoque, with wooden frames, longerons and stringers and a birch plywood skin. The large tail used a duralumin frame with a fabric covering. Polikarpov chose to use a radial engine for his new fighter because the inline engines then available in the Soviet Union were heavier than their radial equivalents and only produced the same power. The extra streamlining allowed by the smaller frontal area of the inline engines didn't make up for the extra weight, and the radial engines were also better able to stand up to battle damage.

The low cantilevered wing was constructed in three sections - a central section built into the fuselage and two outer panels. The wing was built around two stainless steel spars connected by duralumin ribs and tubular struts. The outer panels were connected to the central structure by tubular fittings with threaded outer sections. The wing was fabric covered but with duralumin strips at key points, including over the join between outer and central sections and over the leading edge. The wings had no dedicated flaps, but both ailerons could be lowered at the same time, effectively performing the same role.

The prototype and early production aircraft had a fully enclosed cockpit, with a sliding canopy and a downward opening door on the port side. On many later aircraft the enclosed cockpit was removed and a windscreen replaced it. The aircraft had manually operated retractable landing gear which needed 44 revolutions of its handle to fully retract.

The I-16 was designed to be the smallest and lightest aircraft that could satisfy the demands of the Red Air Force, and was actually slightly shorter and with a narrower wingspan than the I-15 biplane. Its centre of gravity was positioned to improve manoeuvrability, although this did reduce its stability in flight, and at first it was considered to be a difficult aircraft to fly, only suitable for experienced pilots.

The prototype was originally unarmed, but later gained two wing-mounted 7.62mm machine guns, the standard armament of early production versions of the aircraft.

Production of the I-16 began at zavod 39, the factory attached to the TsKB design bureau. This plant was not suitable for the large scale manufacture of series aircraft, and large scale production began at zavod 21 at Gor'ky. Later on large scale production also began at zavod 153 in Novosibirsk. Zavod 21 produced 8,494 aircraft, Zavod 153 produced 1,307, and Zavod 458 another 439, all two-seat trainers.

Zavod 39 was responsible for the first fifty production aircraft, built in 1934 and powered by M-22 engines. After that it only built eight more I-16s - four in 1935 and four in 1936. Of these five were lightened aircraft used by the 'Red Five' aerobatic team and powered by imported Wright Cyclone R-1820F-3 engines (one of which was also used as the prototype for the I-16 tip 5.

Zavod 21 began production with the I-16 tip 4 (type 4), indicating that this was the fourth type of aircraft to be produced at the factory (including the Polikarpov I-5 and the Neman KhAI-1 passenger aircraft, the third earlier type eludes me). The fifty aircraft produced at Zavod 39 are sometimes referred to as the tip 1.

The I-16 was produced in impressively large numbers. Over 1,000 aircraft were built in every year from 1937 to 1941, with a peak of 2,710 in 1940. Production dropped away dramatically in 1942, when the last 83 aircraft, all two-seat trainers, were built at zavod 458.