I-16 tip 12 (I-16P - pushechnyy, cannon armed)
The I-16 tip 12 (or I-16P - pushhyechnyi or cannon armed) was the first version of the fighter to be armed with 20mm cannon. It was developed in 1936 by fitting two 20mm ShVAK cannon to a tip 5 airframe, with the cannon mounted in the wing centre section and synchronised to fire through the propeller arc. Only a small number of tip 12s were produced.
I-16 tip 17
The I-16 tip 17 was the first cannon armed version of the I-16 to be produced in significant numbers. This time the 20mm cannon were mounted further out, beyond the propeller arc. The type also retained the two machine guns in the upper fuselage. Despite the extra weight of the cannon the top speed of the I-16 only fell from 278mph on the tip 10 to 264mph on the tip 17. Twenty seven tip 17s were built in 1938, and 314 in 1939. Production them moved onto the tip 27.
I-16 tip 27
The I-16 tip 27 was the cannon-armed equivalent of the I-16 tip 20, and combined the airframe of the tip 18 with its two 20mm cannon in the wings with the ability to carry slipper-type auxiliary fuel tanks under the wings. Fifty nine were built in 1939, and a combined total of 277 tip 27s and 28s in 1940.
I-16 tip 28
The I-16 tip 28 combined the cannon armament of the tip 27 and the M-63 engine of the tip 24.
The cannon and 12.7mm armed versions of the I-16 were used as ground attack aircraft (notice this does not say 'exclusively') until enough Il-2s were available to replace them. The aircraft could also carry six unguided rockets, which gave it quite a powerful punch in this role, but the I-16 was at best lightly armoured, and so losses were high.
Although the Red Air Force lost huge numbers of I-16s, the Luftwaffe also suffered heavy casualties in the early part of the fighting, many of them inflicted by pilots flying the I-16.
The I-16 quickly faded from the front line. Production of the single seat fighter stopped in 1941, and so losses could only be replaced until stocks ran down. By the end of 1941 the number of I-16s with units on the front had dropped from just over 1,600 at the time of the German invasion to only 240, and by 1 July 1943 only 42 were still in operational use. The I-16 remained in use with units away from the front line for a little longer, but only 42 were still in use in the western part of the Soviet Union at the end of 1943 and in 1944 the aircraft was withdrawn in the west. They remained in use in the east almost to the end of the war, and the 888th IAP operated the type until August 1945, but none were used during the brief but effective Soviet invasion of Manchuria in the last few days of the war against Japan.
http://www.historyofwar.org/articles/weapons_polikarpov_I-16.html
