The Yakovlev fighters were the most-produced fighters of the Soviet Union during World War II. They were the work-horses of the Soviet air forces, like the Spitfires were to Britain and the Bf 109s were to Germany. These fighters all ultimately derived from the I-26 prototype. This was a promising design but, like almost all new aircraft, had a number of problems. Most countries would have corrected the problems before starting production. Stalin's purges, however, caused the Soviet aircraft industry to miss a cycle of aircraft development. As a result, the Soviets went from having fighters equal to or better than those of other major countries in 1936 to having fighters clearly inferior by 1939. To make up for lost ground, the Soviets instituted crash design and development programs. New aircraft went into production before their problems were worked out or, sometimes, even discovered. Instead, the new aircraft were modified directly on the production lines to try to solve their problems. Typically, a new aircraft needed thousands of changes during its first year or so of production.
The Soviets changed their aircraft designation scheme during I-26 development, so the production fighter became the Yak-1 instead of the I-26. The Soviets also designed a stripped-down version of the I-26 design as a trainer, the UTI-26. The designation scheme change meant that the production trained the Yak-7UTI instead of the UTI-26. The Yakovlev factories were busy correcting some of the problems when the Germans invaded in 1941. An improved model, the Yak-3, was planned to go into production in 1941. It was canceled at prototype stage so that the factories would continue to increase Yak-1 production, rather than interrupt production to switch to a new model.
Demand for fighters dramatically increased because of heavy Soviet air losses. The Yak-7UTI was redesigned to be simpler and use fewer parts, becoming the Yak-7V. On their own initiative, members of the Yak-7UTI team also equipped the trainer with full fighter equipment, and this aircraft's performance proved to be only slightly less capable than the Yak-1. This meant that the Yak-7UTI production lines could be use to make fighters as well as trainers, and the fighter version went into series production in 1941 as the Yak-7.
In 1942, the Yakovlev factories continued to improve the Yak-1 and Yak-7. A Yak-7 project in 1942 led to the creation of a new, improved model, the Yak-9. This fighter quickly went into production and remained in production throughout the war, with numerous improvements made over time.
The Yak-1 continued to be improved as well. In 1943, a major effort to "modernize" the fighter led to the creation of the Yak-1M prototype in 1943. The Soviets decided that the changes warranted a new model number, so the Yak-1M entered production as the Yak-3 in 1944. The Yak-3 was the most agile of the wartime Yakovlev fighters, and, like the Yak-9, remained in production for years after the war ended.