It was not until the outbreak of World War II that most Americans came to realize the value of the airplane in modern conflict. As the fighting grew to global proportions, Americans began in particular to appreciate the farsightedness of our Air Corps leaders in developing the long-range bomber. By 1940 the original Boeing 299 or B-17 had grown from a sixteen-ton ship to a giant twenty-two-ton bomber. The new version, the B-17D, was powered with two 1,200-horsepower radial engines, giving it a speed of more than 300 miles per hour. Continual improvements were made on it and by the spring of 1942 a still more formidable member of the _Fortress_ family, the B-17F, was in production. The B-17F was the most powerful bomber yet produced. It was armed with eleven .50-caliber machine guns and manned by a crew of ten. It could carry more than three tons of bombs to targets over seven hundred miles distant. Its oxygen system permitted its crew to fly the _Fortress_ at altitudes above 35,000 feet. With its eleven heavy machine guns in the hands of a perfectly trained crew, the _Fortress_ was capable of defending itself with deadly effectiveness. The first _Flying Fortresses_ went into action with the United States Army on the day after the attack on Pearl Harbor. Although this country had then only a limited number of _Fortresses_, they and their successors quickly began to distinguish themselves on the battlefronts of the world. [Illustration: THE CONSOLIDATED B-24 _LIBERATOR_ IS POWERED WITH FOUR 1,200-HP. 9-CYLINDER RADIAL ENGINES. IT CARRIES A CREW OF TEN FOUR POWER-OPERATED GUN TURRETS, MOUNTS FROM TEN TO FOURTEEN .50-CALIBER GUNS, AND CARRIES FOUR TONS OF BOMBS 1,500 MILES AT A SPEED OF 300 MILES PER HOUR.] [Illustration: THE TWIN GUNNER OPERATES TWO .50-CALIBER MACHINE GUNS AND AN AËRIAL CAMERA TO RECORD RESULTS OVER THE TARGET. TWO-WAIST GUNNERS HANDLE .50-CALIBER GUNS, ONE IN EACH SIDE OF THE FUSELAGE. RADIOMAN HAS ONE .50-CALIBER GUN. TOP TURRET GUNS SHOOT IN A CIRCLE AND OVERHEAD. THE POWER-OPERATED BALL-TURRET, MOUNTING TWO .50-CALIBER MACHINE GUNS, REVOLVES SO THAT THE GUNNER CAN SHOOT DOWNWARD, TO THE RIGHT, LEFT, OR REAR. BOMBARDIER HANDLES TWO .50-CALIBER GUNS IN THE SWINGING CHIN TURRET. PILOT AND CO-PILOT 1,200-HORSEPOWER, SUPERCHARGED, 9-CYLINDER WRIGHT _CYCLONE_ RADIAL ENGINES. NAVIGATOR HAS ONE FLEXIBLE .50-CALIBER GUN MOUNTED IN WINDOWS IN EACH SIDE OF THE NOSE.] By the summer of 1942 _Flying Fortresses_ had begun what was to be the greatest sustained aërial invasion the world had ever known. Starting with a small group of _Fortresses_, the United States Army Air Forces went to work to wreck Adolf Hitler’s “Fortress Europe” and clear the path for an Allied invasion.