From small raids by a dozen _Fortresses_ the number of bombers grew until the raids became huge aërial invasions involving hundreds of bombers and thousands of airmen. That the path for invasion was cleared and victory brought nearer was due in no small measure to our big bombers and the farsighted American airmen who had brought them into being against almost insurmountable obstacles. [Illustration: MARTIN B-26 _MARAUDER_ HIGH-SPEED, TWO-ENGINED, ARMY MEDIUM BOMBER] ARMY ATTACK AVIATION AND TRAINING Although the airplane in World War I had been used mainly as an observation and a plane-to-plane combat weapon, wise American airmen, such as General “Billy” Mitchell, visualized the craft as a means of destroying the enemy’s ability to fight. These men saw his weapons destroyed as they were being built and his transport stopped before it reached the battlefield. As the result of this thinking, our doctrine of air power was established. With this much accomplished, the need for various types of airplanes was clearly defined. It called for three distinct types of warplanes: the long-range bomber, the observation plane, and the pursuit plane. Air strategy was built around the long-range bomber. This was the weapon which would destroy the enemy’s war plants and military establishments on his home grounds. The observation plane was to be used to seek out the enemy’s movements and to locate his installations. As aërial photography was perfected, the observation planes were to be equipped to bring back a record of their findings. These records would establish the targets for the long-range bombers. In the beginning, the pursuit plane was considered a weapon to protect our own military establishments, our cities, and our war plants. Its mission was to intercept any enemy planes attempting to attack us. On the preceding pages we have seen the bomber develop from a single-engined DH-4 into the giant four-engined B-17. This development was the result of the careful study of aërial strategy by our Army airmen. When the big bombers with a range of thousands of miles were built, our strategists saw them as weapons to be used only against an enemy’s most distant military establishments. The smaller two-engined bombers which had once been our long-range bombers [Illustration: NORTH AMERICAN B-25 _MITCHELL_ TWO-ENGINED, ARMY MEDIUM BOMBER THE B-25 MOUNTS EITHER FOURTEEN .50-CALIBER MACHINE GUNS AND ONE 3-INCH CANNON OR EIGHTEEN .50-CALIBER GUNS, EIGHT OF THEM IN THE NOSE.] were delegated to the destruction of targets closer to the battlefronts.