The first world war in the air. From 1914 since today, aviation was the technology that progressed the most. If world war two set the tone, seeing the last biplanes and the first jets, the great war nevertheless saw triple fold speed and engine power increase, almost ten times the range and payload. It was the red carpet unfolded for the first airways, and the birth of all warbirds types at once. The only new concept that will emerge after the war was strategic bombing.
Before WW1 there were already some trials of planes: This was in the Greek-Turkish war, which saw a few armoured cars in action as well. Although in 1911, there was more concerns about staying in the sky for some times than to have anything to do with the military, a few private pilots tried their luck at doing dome recognition from the air, mostly on the Greek side. Aerial reconnaissance indeed was better performed with planes than balloons that were quite obvious targets, tied down to the ground. At that stage, planes were utterly slow, less than 100 kph, an advantage for a mission that need time to study the landscape and enemy positions. It was then more a question of basic imagery intelligence, enemy maneuvers than artillery spotting. These tasks were previously (and still were in 1911) devoted to light cavalry. However already in 1909, the US Army purchased Signal Corps Aeroplane No. 1, and so the air reconnaissance by heavier than air was tested.
But soon a new concept emerged, air attack. There was an early registered balloon attack in 1849 by an Austrian manned hot air balloon above Venice. Later in 1910, an air attack, with dynamite was performed by Didier Masson and Capt. Joaquín Bauche Alcalde, flying for Venustiano Carranza (Mexican revolution). In 1911 Italo-Turkish war, Italian dirigibles bombed Turkish positions on Libyan territory. More famously on November 1, the first heavier-than air bombing was performed by Sottotenente Giulio Gavotti, on Turkish troops. The plane was shot down by rifle fire. In the 1st Balkan war, at the 8 February 1913 Epirus front siege, Russian pilot N. de Sackoff (flying for the Greeks) was also shot down the same way in combat, after making a bomb run on the walls of Fort Bizani. But soon, plane speed would render these ground infantry fire totally useless. Indeed three years later, the average fighter speed was around 150-200 kph, not speaking of the altitude.
The French were actually the first to take aviation seriously on a military standpoint: The Aviation Militaire, organic to the French Army, was formed 22 October 1910. At that stage, the only model in service was the proven Bleriots, sturdy monoplanes, but also Henri Farman biplanes, REP, Sommer, and the great manoeuvers of Picardie were to try and compare these aircrafts performances and those of dirigibles in particular for artillery spotting and setting. As early as june 1910, the first "long range" military transport was performed between Mourmelon camp and Vincennes on a Henry Farman biplane in 2h30. Capt. Charles Marconnet was the unique passenger. At that early stage, brevetted military pilots were called "sapeurs" (Sappers) as their duty was assimilated to artillery personal. In 1912, the British created the Royal Flying Corps, attached to the Army and the next year, the Royal Naval Air Service, attached to the Navy.
In 1914, there were several escadrilles ("squadrons") operating with the French Army, mostly with Bleriot and Farman types, but soon Voisin types as well. The first machine-gun mounted on a plane was on a Deperdussin type 1914 before the war. The MG stood on a tripod above the engine, and the bullets just passed the tip of the propeller. The first air-to-air kill occurred on 5 October, 1914, when a French Voisin equipped with a MG shoot down a German reconnaissance Aviatik. From then on, things would accelerate.
Adolphe Pegoud's loop. He became the first to test a parachute and the first ace.
The Germans had many types built in relatively small quantities, and preferred streamline, liquid-cooled inline engines and profile fuselages to achieve better speeds. However they were keen to reuse ideas and improve innovations. Roland-Garros system with a machine gun firing straight into the armoured propeller blades, which deflected these. Short down, the plane was captured and improved by Fokker, leading to an interrupter gear that enabled to fire between the blades in rotation. These first synchronized machine guns fitted on the E-1 (Eindecker) the first dedicated fighter, and created for some time the first "fokker scourge".
Fokker DR.I, Jasta 26 at Erchin, France. Probably the most agile fighter ever built.
A second one will occur in April 1917, the allies still had numerical superiority but lost technical edge over the Germans, between the fast Albatros and the super-agile Fokker DR-III or triplane, once again copied and improved over the British Sopwith Triplane. Baron Manfred Von Richtofen and his famous "flying circus" became legendary. But soon the allies came back with in particular three new fighters: The French SPAD VII, and the British SE.5 and Sopwith Camel. In 1918, 1st of April, the Royal Air Force was created as the first independent air force.
This only scratched the surface... You have the whole website to get a better idea of this !