While the lineage of the Beretta M9 can trace its way all the way back to The Great War, its common for the pistols designed in the intervening years to be overshadowed by the service of the first and latest models. Most shooters I know think the M9 was a new design out of thin air just for the US military.
The M9, as we know it today, was not the result of a single designer. Instead, it was an evolution on a platform that is now over a century old. The first open slide handguns was the Modelo 1915, which was adopted as the service pistol for Italy during World War I. The blowback design was well-liked, if a little difficult to operate due to springs and relatively poor ergonomics. It was later supplanted by the Modelo 1917 in .32 ACP.
From there, the open slide concept was refined and updated to handle the higher power loads of 9mm. One of the late models, just before the US M9 was the model 92S, which was an update of the first Beretta 92. The most significant change (which many bemoan today) was the movement of a frame mounted safety to the slide mounted safety.
Tim over at Military Arms Channel has his hands on one of the classics. Enjoy the video below.