The Japanese never really embraced submachine guns during and before World War Two. A series of development programs in the 1920s and 30s led nowhere, and there never really seems to have been much motivation behind them. Some small batches of guns were purchased from abroad for units like the Special Naval Landing Force, comprising things like SIG Model 1920 Bergmann guns and Steyr MP34s. Finally in the late 30s, apparently spurred by Japanese experience in the taking of Shanghai, Kijiro Nambu replaced his complex early designs with a simple blowback open-bolt gun chambered for the standard 8mm Nambu pistol cartridge. This was tested and accepted in 1940 as the Type 100.

The early 1940 model of the Type 100 had a distinctive underdog on the barrel shroud for attaching a bayonet, and some examples had bipods or simplistic folding stocks. It wasn’t until 1944 that the design was simplified and production increased - although still not to a level that would be considered significant in any other army. Only about 8,000 of the 1944 pattern guns were made. They had a higher rate of fire (about 800 rpm, compared to 450 rpm on the 1940 pattern), and used a different 30-round curved magazine as well.