In 1937, Mikhail Yevgenyevich Berezin began designing a new large-caliber aircraft machine gun chambered to the 12.7 mm round used by infantry machine guns.
The new design passed factory trials in 1938 and was accepted into service in 1939 under the designation BS (Березин Синхронный, Berezin Sinkhronniy, Berezin Synchronized).
The rate of fire made it well suited for use as defensive armament in aircraft.
While a successful design, BS was not without its faults, the biggest being its cable-operated charging which required considerable physical strength.

Continued development resulted in the improved UB which came in three versions: UBK (Крыльевой, Krylyevoi, for the wings), UBS (Синхронный, Sinkhronniy, Synchronized), and UBT (Турельный, Turelniy, for the turret), with UBS and UBK charged by compressed air.
The UB was accepted into service on April 22, 1941, just two months before the start of the German-Soviet War.
In 1937, Mikhail Yevgenyevich Berezin began designing a new large-caliber aircraft machine gun chambered to the 12.7 mm round used by infantry machine guns. The new design passed factory trials in 1938 and was accepted into service in 1939 under the designation BS (Березин Синхронный, Berezin Sinkhronniy, Berezin Synchronized). The rate of fire made it well suited for use as defensive armament in aircraft. While a successful design, BS was not without its faults, the biggest being its cable-operated charging which required considerable physical strength. 
Continued development resulted in the improved UB which came in three versions: UBK (Крыльевой, Krylyevoi, for the wings), UBS (Синхронный, Sinkhronniy, Synchronized), and UBT (Турельный, Turelniy, for the turret), with UBS and UBK charged by compressed air. The UB was accepted into service on April 22, 1941, just two months before the start of the German-Soviet War.
Berezin UB