The Polikarpov Po-2 (also U-2 before 1944, for its initial uchebnyy, 'training', role as a flight instruction aircraft) was an all-weather multirole Soviet biplane, nicknamed Kukuruznik (Russian: Кукурузник,[3][N 1] NATO reporting name "Mule").

[1] Precise figures are hard to obtain since low-rate production by small repair shops and aero clubs is believed to have continued until 1959.

[6] The prototype of the U-2, powered by a 74 kW (99 hp) Shvetsov M-11 air-cooled five-cylinder radial engine, first flew on 7 January 1928 piloted by M.M.

The unit earned numerous Hero of the Soviet Union citations and dozens of Order of the Red Banner medals; most surviving pilots had flown nearly 1,000 combat missions by the end of the war and took part in the Battle of Berlin.

They typically attacked by surprise in the middle of the night, denying German troops sleep and keeping them on their guard, contributing to the already high stress of combat on the Eastern front.

The usual tactic involved flying only a few meters above the ground, climbing for the final approach, throttling back the engine and making a gliding bombing run, leaving the targeted troops with only the eerie whistling of the wind in the wings' bracing-wires as an indication of the impending attack.

[6][8] Luftwaffe fighters found it extremely hard to shoot down the Kukuruznik because of two main factors: the pilots flew at treetop level where they were hard to see or engage and the stall speed of both the Messerschmitt Bf 109 and the Focke-Wulf Fw 190 was similar to the U-2s maximum speed, making it difficult for the fighters to keep a Po-2 in weapons range for an adequate period of time.

[6][9] The success of the Soviet night harassment units inspired the Luftwaffe to set up similar Störkampfstaffel "harassment combat squadrons" on the Eastern Front using their own obsolete 1930s-era, open cockpit biplanes (most often the Gotha Go 145 and Arado Ar 66 biplanes) and parasol monoplane aircraft, eventually building up to larger Nachtschlachtgruppe (night attack group) units of a few squadrons each.

[11] The fabric and wood construction of the airplane made it extremely vulnerable to catching fire when hit by tracer rounds, resulting in a Russian nickname of Kerosinka, or kerosene lantern.

A significant number of Po-2s were fielded by the Korean People's Air Force, inflicting serious damage during night raids on United Nations bases.

[14] UN forces named the Po-2's nighttime appearance Bedcheck Charlie and had great difficulty in shooting it down – even though night fighters had radar as standard equipment in the 1950s.

On 16 June 1953, a USMC AD-4 from VMC-1 piloted by Major George H. Linnemeier and CWO Vernon S. Kramer shot down a Po-2, the only documented Skyraider air victory of the war.

The Po-2 is also the only biplane credited with a documented jet-kill, as one Lockheed F-94 Starfire was lost while slowing down to 161 km/h (100 mph) – below its stall speed – during an intercept in order to engage the low flying Po-2.