The first Air-Launched Cruise Missile was the German V-1 "Buzz-Bomb" which was employed as such in the waning days of World War II. Originally designed to be launched from a steam-driven catapult (similar to the system employed to launch aircraft on some modern aircraft carriers), the success of the D-Day landing in Normandy on June 6, 1944, put these plans in serious jeopardy. The catapult systems were easily located and destroyed by patrolling Allied aircraft; indeed, every launching platform had been either destroyed or disabled prior to Operation Overlord. The V-1 did not fly until June 12, 1944. Very quickly the Germans built new launch sites at Par-de-Calais, but these too were vulnerable to assault from the air. The V-1's limited range (approximately 200 miles) meant that further Allied advances might place strategic targets like London entirely beyond the reach of the flying bomb. Another launching solution was required.
Early, proof-of-concept tests in air launch of the V-1 had shown that this was a feasible though less-than-desirable method of deploying the weapons. With the remaining land-based launchers in danger of capture or destruction the German forces turned to this method once again. Beginning in July of 1944 Heinkel He 111 bombers, each with a single V-1 missile mounted inboard of their left engine, took off from airfields in Germany and flew out over the North Sea. After reaching a specified distance from their target (generally London) the weapon was released from the aircraft and sent off to wreak havoc.
Or probably not. Roughly a quarter of all V-1s crashed before they reached land; the air-launched weapons also had the annoying habit of falling into the North Sea, likely due to problems with the craft's pulse jet. Of the remainder, half were destroyed by Allied fighters or land-based anti-aircraft batteries. V-1 air launches were also hampered by the growing Allied air presence in continental Europe, which limited the number of He 111 aircraft available for deployment of the buzz-bomb; of the air-launched V-1s targeted at London on a daily basis it is estimated that at most a handful successfully reached the city. The Germans continued air-launch of the V-1 until January of 1945, when the Allies' growing air superiority over Europe made the practice untenable. Land-launch of the missile ended in March of the same year.