Designed and built by the Nambu Arms Manufacturing Company, the Type 100 was a robust, if unremarkable, submachine gun that was first delivered to the Imperial Army, in 1942. Japan was surprisingly late to introduce the submachine gun to its armed forces.

The Type 100 was a well made gun, but the 8x22mm Nambu round was underpowered, roughly the equivalent to the .380 ACP. Atypically for a submachine gun but typical of Japanese small arms, a bayonet lug was fixed under the barrel.

Despite its shortcomings and complexities, the Type 100 had a high quality chrome plated barrel to aid cleaning and reduce wear. Some models also featured a bipod or a complicated muzzle brake.