Blame Clément Ader. Having failed to achieve flight, he abandoned active testing, and turned to lobbying about aviation instead. Writing in 1907, he prophesied that a failure to invest in air power would leave France facing a doomsday scenario: great Anglo-German air fleets menacing the capital itself. 'These airborne cohorts will fly methodically over the ten main arrondissements of Paris, bombing as they go, sparing neither museums nor historic monuments, and dropping on average four or five bombs on each dwelling.' Naming Britain as a potential enemy so soon after the conclusion of the Entente Cordiale cannot have helped him get an official hearing, but his lurid writings certainly grabbed the attention of excitable politicians and excitable newspapers.
At this early stage of aircraft development the sticking point was weak engines which could not develop sufficient power to carry both crew and a large offensive payload. What was billed as an attempt to encourage manufacturers to create such aircraft, a competition held by the War Ministry at Reims in 1911, failed because the competition conditions only specified the size of payload that existing aircraft were capable of carrying, rather than any improvement, and so the winner ended up being the fastest (but which, even according to the manufacturer, 'handled like a flat iron'). The expense and fragility of early aircraft made many senior officers reluctant to devote money to their development, when it could be better spent on artillery and machine guns, considering aviators as 'buffoons or acrobats from whom nothing serious could be expected.'
For their pre-war supporters, balloons represented the way forward. As one of the original components of military aviation, balloons were a proven technology, unlike the new-fangled aircraft. Although slower than aircraft, dirigible balloons could carry a large payload at a greater altitude, with a much greater range. After war broke out, the Adjudant-Vincenot and the Fleurus I both undertook reconnaissance / bombing raids into the Germany, as far as Sarrebourg and Trier. However, balloons always remained vulnerable to ground fire: on 24 August 1914, after undertaking bombing raids as far as Louvain, the Dupuy-de-Lôme was returning to French-held territory over Reims when it was shot down by the city's (French) garrison. When news reached GQG that the Germans had lost four Zeppelins in the same period, the dirigible fleet was grounded. Missions were eventually restarted on 2 April 1915, but only at night.
14 August 1914, two pilots from MF16, Lieutenant Antoine Cesari and Corporal Roger Prudhommeaux, undertook the first French air raid, when each dropped a modified 155mm shell on the zeppelin hangar at Metz-Frascaty. For GQG, 'aviation is an arm of service, [and] clearly an offensive one, whether that be in pursuing enemy aircraft, or in bombing enemy troops, camps and fortifications. It can operate independently at long or short range, or attack in liaison with other troops.' From here, by using aircraft types with a long operational range, 'capable of destroying or blocking enemy railway lines some 200 … to 250 kilometres behind the front,' aviation could serve strategic aims. 'We must free ourselves of short-term thinking, widen our horizons and extend our ambitions to obtain every possible advantage from the aircraft which will be available to us.' Raids on German factories would disrupt the enemy's war effort, give French official communiques something good to report on, and so bolster French morale.
Major Louis de Goÿs, increasing to four groups of four squadrons by the spring. De Goÿs had been an enthusiastic follower of Ader's vision - when Turkey joined the war, he was calling for a massive aerial armada to frighten the Turks out of the war. The first strategic raid was against the Badische Anilin chemical works in Ludwigshafen in May 1915 (where de Goÿs was shot down and captured - 'Sudden engine failure. I can't tell you how disappointed we were'), then a second attack, in retaliation for a zeppelin raid on Paris, on Karlsruhe in the following month.
Pictures: Ader (from Wikipedia); the dirigible Adjudant Vincenot (from Delcampe); a Voisin 3, mainstay of the French bomber squadrons, at the Le Bourget museum (my picture); Lieutenant Cesari (from Delcampe); Major de Goÿs, the crews of Groupe de Bombardement Nr.1, and bombs landing on Karlsruhe (from contemporary magazines)