The FN P90 submachine gun (SMG) was developed in the late 1980s as a personal defense weapon for the troops whose primary activities does not include small arms, such as vehicle and tank crew members, artillery crews etc. Standard pistols and submachine guns chambered for pistol rounds were proved ineffective against enemy soldiers, wearing body armor; Therefore FN designers first developed a new round with enhanced penetration, initially known as SS90. To achieve necessary high penetration while keeping recoil impulse low, FN used a small-bore approach, creating a round that looked much like the scaled down 5,56NATO round. It must be noted that similar concepts were tried in other countries, most notably in USA, several decades before FN; for example, US Army tested M1 carbines chambered for .221 Johnson Spitfire round back in late 1950s; later on, Colt produced its .22 SCAMP and 5,6x30 MARS ammunition for special SCAMP machine pistol and MARS "mini assault rifle" (a scaled-down M16 rifle) respectively. It must be noted that 5,6x30 MARS round was in a sense a direct predecessor to 5,7x28 FN SS90 round, although the latter featured slightly lighter and faster bullet. By late 1980s a concept of a small-bore, low-impulse "personal defense weapon" (PDW) with good accuracy and lethality at ranges of up to 200-250 meters was well established, although there were no weapons adopted for service yet. FN decided to follow this concept and to create its own PDW using clean sheet approach. Basic ideas used for this development, designated as "Project 9.0", included the following: minimal size and weight of weapon; large magazine capacity; complete ambidexterity; ease of use and maintenance.
To save on size and weight, FN designers put new weapon into compact and lightweight stock of bullpup layout, made of impact-resistant polymer. The high-capacity magazine also was made from semi-translucent polymer, and holds 50 rounds in two rows. To made loaded weapon as compact as possible, FN designers followed the idea of American designer Hall, and placed the magazine above the barrels, with cartridges stored in horizontal position with bullets pointing to the left. While Hall system employed a rotary feed unit, operated by the bolt, to put new cartridge in line with the barrel, FN designers incorporated a stationary helical ramp into each magazine, which rotates cartridge for 90 degrees prior to placing it into feed lips. Complete ambidexterity was achieved by using ambidextrous controls (including dual charging handles and dual back-up open sights), and bottom ejection. Finally, simplicity of aiming was achieved by use of integral reflex type collimating sight and integral laser aiming module (LAM).
Resulting weapon appeared in around 1990 as FN P90 personal defense weapon, along with improved 5,7x28 SS190 ammunition, which replaced polymer-cored bullets with heavier dual-core (steel / aluminum) bullets with better penetration against body armor. Several other types of ammunition were developed for this weapon, including tracer, subsonic ball and soft-core training ball.
First sales of P90 were made to Saudi Arabia in early 1990s; today it is believed that FN sold more than 20 000 of P90's to a wide variety of law enforcement agencies and military special operation units worldwide, including US Secret Service, Austrian Army rangers, Dutch BBE special operations forces, Belgian Army and others. Most interesting fact about adoption of P90 is that so far it has been adopted for the role, directly opposite to its original niche of "personal defense weapon". In fact, most services and agencies that adopted P90 use it for offensive roles, as a specialist or even a primary weapon for various assault teams, and other "professional small-arms users", as opposed to military personnel which primary functions do not include use of small arms.
In around 1995 FN supplemented the P90 with pistol, firing the same 5,7x28 ammunition, designated as FN Five-seveN. Recently, FN also introduced a civilian version of P90, designated as PS90 carbine. This is a self-loading weapon with longer barrel (408 mm / 16").
FN P90 personal defense weapon is blowback operated, selective-fired weapon which fires from closed bolt. The firing is controlled by a removable trigger unit with conventional hammer. A manual safety is located directly below the trigger. Magazine lies at the top of the weapon, feeding from front-to-back, with spiral ramp built into the "rear" part of the magazine. Spent cartridges are ejected straight down through the chute, which exits just behind the pistol grip. The standard sighting equipment includes a non-magnifying collimating sight with "ring and dot" illuminated aiming reticule. Back-up open sights are provided at either side of the primary collimating sight. The so-called P90 USG version is provided with two additional Picatinny rails at either side of the collimating sight base; FN also offers a version with no standard sighting equipment; user has to make its own choice of day and/or night sights and additional equipment, which can be installed on three Picatinny rails - top, left and right. This version is designated as P90 TR (triple rail). The front part of the forward handgrip on P90 is shaped as hand protector, and it can contain integral laser aiming module, which sends either visible or IR laser beam to mark the intended target. For special missions P90 can be fitted with a special silencer, which is used in conjunction with special subsonic ammunition.