For years the Italian Carabinieri wielded the Beretta Model 12 SMG quite successfully. This was a stamped steel SMG that fired from an open bolt and was designed in 1959. For decades they used the Model 12, and when they needed a new SMG, they basically wanted a more modern Model 12…which is exactly what Beretta provided.
The Beretta PMX is the product of those improvements. The gun was also entered into the Army’s SMG contest, but lost out to the Swiss APC9K.