This is the first in a series of nodes presenting a not terribly serious history of military aviation in the United States. The next node in the series is Army Air Corps.
In the beginning, weapons had short ranges and it was pretty easy to coordinate battles and wars by sending messengers on horses hither and yon. Later, wars got bigger, and battles got complicated, and you could shoot people from five hundred yards away. Fortunately, people are inventive, and some bright fellow had come up with a method of signaling involving flags. Some other bright fellow had invented a thing called the telegraph. Unfortunately, you couldn't just yank some illiterate yokel out of the ranks and hand him a couple of signal flags to make with the wig-wags, much less bash out some esoteric Code on a telegraph key. You had to train literate people to do these things, and so the Signal Corps was born.
Technology continued to advance, and the military found uses for things like telephones, the heliograph, and carrier pigeons, to say nothing of radio. This ongoing fascination with technology made the Army's Signal Corps a natural to get involved with this newfangled aeroplane the Wright Brothers had come up with, which in fact it did. At this point, things started to get a bit strange and diverge from the mission of the Signal Corps. People quickly realized that you could look down from the aeroplane and see the ground, which had obvious implications for the cavalry, who up to that point had pretty much had the corner on the reconnaissance business. People also realized that you could drop things out of aeroplanes, like grenades, and to prevent this, other aeroplane drivers (now called pilots) started taking a pistol, then rifles, and eventually machine guns, up into the air for the express purpose of shooting at other pilots and their aeroplanes...and making life miserable for any fools in balloons that might be hanging around the battlefield, while they were at it.
Airplanes by this time had gotten considerably less flimsy than the original Wright Brothers contraption, and could carry quite a bit of weight. Some of that weight came to be devoted to bombs, which sort of got the Signal Corps into the artillery business, but nobody minded too much since there were thousands and thousands of officers and men in the Artillery Corps and not too many in the Signal Corps even with all these aviators swanning about. Airplanes had also become specialized: there were bombers to drop things on people, observation planes to bring back word of what things needed bombs dropped on them, and pursuit planes to shoot down enemy bombers & observation planes that were trying to do their thing. The whole business was very romantic, very reminiscent of medieval knights saddling their steeds and riding out to battle, and people like Eddie Rickenbacker and Manfred von Richthofen (The Red Baron!) became very famous for zooming around in their pursuit aeroplanes shooting other pilots out of the sky.
And the advance of technology continued, at a higher speed now since this was wartime and everybody wants to get an advantage over the other guy - like, being able to fire one's machine gun through the airplane's propeller without shooting said propeller into splinters, after which one generally fell out of the sky and died in a fiery crash. Various ideas were tried - sticking the propeller in back (too slow), sticking the gun on the top wing so as to avoid the propeller issue altogether (too clumsy), or just putting armor plate on the propeller (too risky). Eventually Dutch aviation genius Anthony Fokker invented the interruptor gear, which stopped the machine guns from firing when the propeller obstructed the barrel. This inaugurated the point and shoot interface for pursuit pilots, and increased the number of pilots being shot out of the sky, dying in fires, and otherwise contributing to other pilots' kill totals.
While all this innovation was going on, Signal Corps aviators were thinking Serious Thoughts and conceiving Grand Theories about how the airplane was going to change warfare. Being military aviators, of course, they didn't for a minute entertain the silly notion that airplanes would make warfare obsolete (take that, Mr. Wells!) but concentrated on how airplanes could make war shorter by destroying the enemy's ability to make war in the first place. This concentration also led them to the firm conviction that the Navy and Army needed less money for conventional weapons, since in the future aircraft would make all those hideously expensive battleships and huge formations of infantry redundant. Shooting off his mouth about this sort of thing eventually got Billy Mitchell cashiered, but by then there were enough officers in the Air Service who felt the same way (and perhaps more importantly, politicians who supported them) that the Army split the Service off to form the Army Air Corps in 1926.
At this point we lose interest in the Signal Corps, which continues to fiddle with radios and other communications technology to the present day, not that anyone really cares about any of that nonsense until it stops working.