Then a more brutal war entered the picture, and the major powers were soon vying for control of the air. World War I's flying machines, which served at first only for reconnaissance, were soon turned into offensive weapons, shooting at each other and dropping bombs on enemy positions. The fighting in the skies was matched by a fierce competition among aviation engineers on both sides. When one side built more powerful engines, the other countered with sleeker streamlining; the development of bigger planes that could drop heavier bombs was countered by improved maneuverability to get the upper hand in dogfights. With each adjustment that worked, aviation took another step forward.

Some of the most significant developments involved the airframe itself. The standard construction of fabric stretched over a wood frame and wings externally braced with wire was notoriously vulnerable in the heat of battle. Some designers had experimented with metal sheathing, but the real breakthrough came from the desk of a German professor of mechanics named Hugo Junkers. In 1917 he introduced an all-metal airplane, the Junkers J4, that turned out to be a masterpiece of engineering. Built almost entirely of a relatively lightweight aluminum alloy called duralumin, it also featured steel armor around the fuel tanks, crew, and engine and strong, internally braced cantilevered wings. The J4 was virtually indestructible, but it came along too late in the war to have much effect on the fighting.

In the postwar years, however, Junkers and others made further advances based on the J4's features. For one thing, cantilevering made monoplanes—which produce less drag than biplanes—more practical. Using metal also led to what is known as stressed-skin construction, in which the airframe's skin itself supplies structural support, reducing weighty internal frameworking. New, lighter alloys also added to structural efficiency, and wind tunnel experiments led to more streamlined fuselages. Step by step, a more modern-looking airplane was taking shape.