The submachine gun contains several innovative features, including the proprietary ammunition. It uses the new, 5.7x28mm cartridge which has a greater penetrating capability, lethal range and flatter trajectory than pistol caliber cartridges, such as the standard 9x19mm Parabellum. Initially the weapon used 5.7x28mm ammunition with the SS90 cartridge (lightweight, roundnose, jacketed projectile with a polymer core), as well as tracer, training (reduced range), sub-caliber (increased velocity and effective range of 250 m) and blank ammunition. The first prototype firing this ammunition was completed in October 1986, and over 3000 submachine guns had been produced until 1993 in a low-rate initial production run. Simultaneously, FN chose to revise the ammunition, with the intention to use it in a planned semi-automatic pistol of the same caliber (known today as the Five-seveN). The new cartridge, designated the SS190, has a more conventional full metal, plated steel jacket, lead core and steel/aluminum penetrator. Several other projectiles were also developed for the new cartridge, including the L191 tracer round, a subsonic SB193 bullet and blank ammunition. This modified version of the P90, adapted for use with the new ammunition was introduced in 1993.

The P90 is considered a PDW (Personal Defense Weapon), and is designed as a compact but powerful firearm for vehicle drivers, operators of crew-served weapons, support personnel, special operators and anti-terrorist units.