The PM-9 was designed in the early 1950s by Louis Bonnet de Camillis. Fenner Achenbach, de Camillis' financier, sold the production rights to the newly-reestablished Erma firm in West Germany. Since Erma's original factories were captured by the Soviets during World War II, the new Erma factory in Dachau lacked much of the industrial machinery required for mass production and thus their new designs, including the PM-9, were largely constructed by hand. Erma used the production of the PM-9 as a means to train new employees and apprentices.
Marketing of the PM-9 was handled by a Grenoble-based company, Societé Pour l’Exploration des Brevets MGD, which had been established to exploit new patents. MGD began marketing the weapon in 1954, but despite their best efforts, the PM-9 sold poorly due to its high price and technical complexity, and production ceased only a year later in 1955. Very few examples of the PM-9 were ever made.
The PM-9 operated on the blowback principle but used a unique flywheel mechanism to delay the bolt. The bolt was attached by a guide rod to a large flywheel in the weapon's rear. When the bolt sprung back upon firing, the wheel would rotate 180° backward, bringing the bolt with it, and then push back forward 180°, bringing the bolt back to its original position. The system allowed the weapon to be kept at a short length, but was mechanically complex and difficult to repair in the event of a malfunction.
Like many other French-designed submachine guns of the period, the PM-9 featured not only a folding stock but also hinged magazine housing that retracted under the barrel. With both the stock and magazine folded, the PM-9 was very handy and could be carried or stored with ease.