In the early 1930s, biplanes of various types and purposes formed the basis of the air forces of many countries. They had their advantages, including good stability in the air, high maneuverability, and reliability. This allowed many of them to remain in service with the air forces during the 1930s and 1940s. They also took an active part in the fighting of World War II. The Japanese Imperial Army's Ki-10 fighter was one of the best biplane fighters of its time, and during WWII, it was used in training aviation units. The U-2 aircraft was one of the most mass-produced and famous aircraft in the history of world aviation; the U-2VS modification was effectively used as a light night bomber. The Heinkel He 51 biplane became the first production fighter of the German Luftwaffe. The prototype aircraft made its first flight in the summer of 1933, and in 1935, mass production began. On the eve of WWII, these aircraft were transferred to training units.