The Lahti L39 was the Finnish answer to the need for an anti-tank rifle, developed just before the Winter War. The rifle was created by noted Finnish designed Aimo Lahti, who had pressed for it to use a 13.2mm cartridge. However, arguments for using a 20x138B cartridge won out, based on hopes to use that cartridge in both antitank and antiaircraft roles, as well as testing that showed the 20mm projectile to have greater terminal effect.

The L39 was not available for use in the Winter War (having been adopted barely 2 months before the Russian attack), but was used extensively in the Continuation War. While improved tank armor quickly became thick enough to protect against the round, it was used for a variety of anti material roles, attacking machine gun positions, bunkers, light vehicles, and more. In 1944, an anti-aircraft version was also produced, firing in full auto and using 15-round magazines.

Today, ammunition is available from a few companies, typically using lathe-turned new cases and surplus 20mm Vulcan projectiles.