Nikolai Polikarpov designed the Po-2 to replace the U-1 trainer but ended up making a plane that became a bomber, a fighter, a trainer, an observation craft
The plane was also called the U-2. But it first flew in 1928 and flew in air forces around the world until 1959. It was also used for mail, transport, military officer aircraft, supply aircraft and medevac. It had great hot day high altitude take-off rates because it was a biplane.
There has never been an aircraft produced in greater numbers, but the best guess is that there over 40,000 copies made.
From Wikipedia: After first trials of arming the machine with bombs in 1941, from 1942 it was adapted as a light night ground attack plane. Wehrmacht troops nicknamed it Nähmaschine (sewing machine) for its rattling sound. The material effects of these missions was mostly insignificant, but the psychological effect on German troops was much more noticeable. They typically attacked by complete surprise in the dead of night, denying German troops sleep and keeping them constantly on their guard, contributing yet further to the already exceptionally high stress of combat on the Eastern front. Their usual tactics involved flying only a few meters above the ground, rising for the final approach, cutting off 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. Luftwaffe fighters found it extremely hard to shoot down the Kukuruznik because of three main factors: the rudimentary aircraft could take an enormous amount of damage and stay in the air, the pilots used the defensive tactic of flying at treetop level, and the stall speed of both the Messerschmitt Bf 109 and the Focke-Wulf Fw 190 was similar to the Soviet craft’s maximum cruise speed, making it difficult for the newer aircraft to keep a Po-2 in weapons range for an adequate period of time. The U-2 was known as the plane used by the 588th Night Bomber Regiment, composed of all-women pilots and ground crew. The unit became notorious for its daring low-altitude night raids on German rear-area positions, with veteran pilots Katya Ryabova and Nadya Popova on one occasion flying 18 such missions in a single night. The women pilots observed that the enemy suffered a further degree of demoralization simply due to their antagonists being female. As such, the pilots earned the nickname “Night Witches” (German Nachthexen, Russian Ночные Ведьмы/Nočnye Ved’my). 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 at the end of the war and had taken part in the Battle of Berlin.
It flew at night. It flew in the day. It bombed and transported and saved and resupplied and basically was the single most successful aircraft ever built.
Sources and Citations:
Velek, Martin. Polikarpov U-2/Po-2 (English Edition). Prague, Czech Republic: MBI, 2002. ISBN 80-8624-02-7.