It was renowned watchmaker Cartier who made the first pilot watch and helped lay the building blocks for the wristwatch revolution that would arrive after the war. His friend, the Brazilian ace Alberto Santos Dumont, needed a hands-free timekeeping solution. Cartier set himself to the task. Barnstorming met brainstorming, and the result was the Santos, the first in a long line of pilot watches. The year was 1911 - aviation and pilot watches were still in their infancy, but things would soon take off.
Pilot watches are really aerial chronographs. A chronograph is a watch with additional stopwatch functionality. Pilots needed - and need - to know not just what time it is but how long they've been in the air or on a certain course, at a certain altitude, etc. So already, a pilot watch has to be something special.
Pilot watches must also perform within a certain range of accuracy. Lose too much time and calculations of position or remaining fuel will turn out wrong. Pilot watches also have to deal with differences in temperature and pressure caused by altitude, as well as magnetic forces.
Compared to diving watches, though, pilot watches don't really have that many special features or requirements that set them apart from more standard-issue chronographic watches. They were often similar to the sort of watches that militaries around the world required for general purpose. Still, as the preferred companions of historical aviators, they have a special cache.
We take it for granted that our watches are accurate and remain accurate - our mindset towards time is something like set it and forget it. The original pilot watches, though, were, like other watches, not as precise and reliable as our digital companions. In addition, they had to contend with conditions literally above and beyond those on the ground. That meant that pilot watches need to be easily adjustable so that pilots could be sure that they were still precisely following "real" time.
How'd that work in practice? From the ground, minutes would be sounded off via radio based on the readings of a highly accurate clock back at base. A bezel displaying the seconds was separate from the minutes and hours. It was adjustable, meaning that pilots could use the radio broadcast to sync their seconds display with that of the ground clock without having to adjust the rest of the mechanical readouts on their pilot watches. Since the watches were reliable enough that the minutes, let alone the hours, wouldn't get out of sync, this method ensured that pilots had a simple way of making accurate calculations while in the air. The process of adjusting the seconds is called "hacking" and the seconds on an old-style pilot watch are "hack seconds".