When the armistice that resolved World War I was signed, French war hero Marshal Ferdinand Foch called it a 20-year truce. Unfortunately, he was correct and as the scars of the First World War began to heal, nations began preparing to fight, and win, the next one. Aircraft structures and technologies were rapidly changing and the air fleets of the world’s militaries were working to incorporate every change. Developments such as cantilevered wings, metal construction, variable-pitch propellers, retractable landing gear, engine cowls, de-icing systems, gun turrets, air-cooled engines, and bombsights were incorporated into warplane designs throughout the 1930s, ensuring that each nation would have the deadliest plane. When the United States declared war in late 1941, the prototype for every airplane that saw combat during the war had already been flown. The military was prepared for a new kind of air war to begin, fought with faster, more manoeuvrable airplanes with greater range and altitude.

After the war, the world was slow to begin rearming itself. In all nations, support for military spending was unpopular. Germany, prohibited from building airplanes under the Versailles Treaty, spent the 1920s in a national fervour over gliding. The French, having fought World War I with one of the largest and most successful air forces, nationalized its aviation industry in 1938, destroying it for lack of competition. The United States maintained a strong isolationist policy and continued its tradition of low military spending during peace. The newly created Royal Air Force in Great Britain competed with the other military services for a share in the shrinking military budget. The large amounts of war surplus planes made it seem as if new airplanes weren’t needed. As an economic depression struck the world in the 1920s, military spending during peacetime quickly seemed extravagant.

But World War I had created a worldwide rage for aviation and pilots. It drove the civilian aviation industry. And it also fuelled a craze for races, trophies and records. The world’s airmen seized the opportunity to promote air power and to spend on new technology without appearing to be preparing for war. The U.S. Navy actually had a designation for racing aircraft, until Congress objected. Curtiss built a series of navy racers for the Schneider races, developing fast and manoeuvrable seaplanes. The English entries in the racing series by the Supermarine Company led eventually to the Spitfire fighter. Even the airplanes used by civilians for these races were eventually adapted for military purposes. In 1938, the Seversky pursuit plane, flown by a civilian, won the Bendix Race. After the race proved its abilities, the air corps placed an order with the company and it became the Republic P-47 Thunderbolt, the most produced plane of the war. The pursuit of records, in addition to advancing technology, provided the militaries with valuable operations and logistics experience: the U.S. Air Service’s around-the-world flight by four Douglas World Cruisers in 1924 not only gave the air service publicity, but the six-month flight required support systems around the globe to be set up, including some coordinated with the navy.

In the mid-1920s, Germany began moving toward rearmament, nationalizing the leading airline, Lufthansa, using it to train pilots and begin an aviation industry. With the first sizeable showing of the National Socialist (Nazi) party in the national elections of 1930, the rebuilding of the Luftwaffe became top priority. The Versailles Treaty had mandated the destruction of the entire air fleet. So although this meant starting from scratch, it also meant that the process was unencumbered by war surplus or unusable aircraft. The arms race began to take shape, as other nations started to build fleets to protect themselves. World War I had taught everyone the importance of air superiority. And achieving air superiority required having better planes. These planes had to be not only the best, but they needed to be mass producible and easy to repair in the field. Aircraft development began to focus on two different types of aircraft: fighters and bombers.

Fighter aircraft had to be fast, yet manoeuvrable. The new technological developments began to drastically change the appearance of the airplanes. The Boeing P-26 debuted in 1934 and was the last fighter to feature an open cockpit, which had proved impractical with the new, higher speeds. It was quickly phased out and only saw combat with the Philippines Air Force. In England, the Bristol Bulldog was the last biplane to be built in large numbers by a major air force, but it too soon was abandoned.

 
The Republic P-47 Thunderbolt, the most produced plane of World War II, developed from a Seversky pursuit plane that won the Bendix Race in 1938.

By the mid-1930s, all-metal monoplanes began appearing. The British introduced a pair of fighters, the Hawker Hurricane and the Supermarine Spitfire. The Hurricane was England’s first airplane to feature the Rolls Royce Merlin engine and to have machine guns installed in the wings away from the propeller so that a constant stream of bullets could be fired at the opponent. The Spitfire, with eight machine guns and a top speed of 355 miles per hour, was extremely manoeuvrable, even at high altitude.  The German’s response to the Spitfire, called a "toy" by the German air attaché, was the Messerschmitt Me.109 (sometime called Bf.109). The Me.109 had debuted during the Spanish Civil War, which had allowed it to be tested and modified under combat conditions. It had the smallest possible airframe but held a Mercedes-Benz liquid-cooled engine, Germany’s most powerful. With a top speed of 342 miles per hour (550 kilometres per hour) and the ability to perform swift climbs and fierce dives, the plane was deadly.

 
The Hawker Hurricane was England's first airplane to feature the Rolls Royce Merlin engine and to have machine guns installed in the wings away from the propeller so that a constant stream of bullets could be fired at the enemy.

Other nations also developed high-level fighter airplanes. Poland developed the PZL P.11, which set a world speed record in 1934. The Soviets had the Polikarpov I-16, which also had debuted during the Spanish Civil War. And in Asia, the Japanese quietly built up an air force to achieve the dream of an empire, featuring a long-range fighter that was the best in the Pacific. The Mitsubishi A6M Reisen Zero was designed to be a long-range bomber escort with a high ceiling, manoeuvrability, and speed. Unfortunately, in saving weight for the long distance, protective armour for pilots had to be sacrificed.

The building of bomber fleets reflected the general acceptance by military strategists of bomber theory. To be successful, bombers needed to have long-range capacity and be able to carry heavy bomb loads, which were now kept inside the planes in bomb bays rather than hanging off bomb racks. Protection was provided by machine guns on board, which were now added until bombers began looking like flying porcupines. The development of revolving gun turrets allowed one gun to cover the range of several, reducing the total number needed. The planes were powered by multiple engines.

 
The Boeing B-17 Flying Fortress first flew in July 1935. This four-engine bomber could carry up to 17,600 pounds of bombs and hold as many as 12 guns. It would become the backbone of the U.S. strategic bombing campaign in World War II.

In 1934, the United States introduced the Martin B-10 bomber, the first all-metal, twin-engine monoplane bomber. Although it flew successfully between Washington, D.C., and Alaska, the air service still lobbied for a more powerful bomber. The Boeing B-17 Flying Fortress first flew in July 1935; a four-engine bomber that could carry up to 17,600 pounds (7,983 kilograms) of bombs, held as many as 12 guns, and was destined to become the backbone of the U.S. strategic bombing campaign in the war.

Germany had developed the Junkers Ju.52 to be its bomber, but service in the Spanish Civil War showed it to be inadequate. The development of the Heinkel He.111 was then undertaken, a twin-engine bomber that flew at the beginning of the war. The Royal Air Force, founded by Hugh Trenchard, one of the prophets of aerial bombing, built a wide array of bombers, including the Avro Lancaster, the Wellington, the Stirling, and the Halifax. The Soviets built only one heavy bomber, the Tupelov TB-3, which had started the 1930s as the world’s best but by 1941 was obsolete. And the successes of the Italian Savoia-Marchetti S.M.79, a trimotor, led it to be as popular with the Italians as the Spitfire was to the English.

By 1939, the air forces of the world had grown strong in numbers and sleek in design--old fabric covered biplanes had been replaced by fast metal monoplanes. Germany and England had already begun testing jet engines. A new and deadly air war was beginning.