Air warfare was a major component in all theaters of World War II and, together with anti-aircraft warfare, consumed a large fraction of the industrial output of the major powers. Germany and Japan depended on air forces that were closely integrated with land and naval forces; the Axis powers downplayed the advantage of fleets of strategic bombers and were late in appreciating the need to defend against Allied strategic bombing. By contrast, Britain and the United States took an approach that greatly emphasized strategic bombing and (to a lesser degree) tactical control of the battlefield by air as well as adequate air defenses. Both Britain and the U.S. built substantially larger strategic forces of large, long-range bombers. Simultaneously, they built tactical air forces that could win air superiority over the battlefields, thereby giving vital assistance to ground troops. The U.S. Navy and Royal Navy also built a powerful naval-air component based on aircraft carriers, as did the Imperial Japanese Navy; these played the central role in the war at sea.R.J. Overy, The Air War: 1939–1945 (1980 ) ch 1

Pre-war planning Before 1939, one side (Japan) operated under largely theoretical models of air warfare. Italian theorist Giulio Douhet in the 1920s summarised the faith that airmen during and after World War I developed in the efficacy of strategic bombing. Many said it alone could win wars, as "the bomber will always get through". The Americans were confident that the Boeing B-17 Flying Fortress bomber could reach targets, protected by its own weapons, and bomb, using the Norden bombsight, with "pickle barrel" accuracy.Stephen Lee McFarland, America's pursuit of precision bombing, 1910–1945 (1995) p 68 Japanese aviation pioneers felt that they had developed the finest naval aviators in the world.

Air forces

Germany: Luftwaffe

The Luftwaffe was the aerial warfare branch of the Wehrmacht. Under the leadership of Hermann Göring, it was able to learn and test new combat techniques in the Spanish Civil War. The war also led to greater emphasis on anti-air weapons and fighter aircraft due to their ability to defend against enemy bombers.Williamson Murray, Luftwaffe: Strategy for Defeat, 1933–1945 (1985) Its advanced technology and rapid growth led to exaggerated fears in the 1930s that helped to persuade the British and French into appeasement. In the war the Luftwaffe performed well in 1939–41, as its Stuka dive bombers terrified enemy infantry units. But the Luftwaffe was poorly coordinated with overall German strategy, and never ramped up to the size and scope needed in a total war, partly due to a lack of military aircraft production infrastructure for both completed airframes and powerplants when compared to either the Soviet Union or the United States. The Luftwaffe was deficient in radar technology except for their usable UHF and later VHF band airborne intercept radar designs such as the Lichtenstein and Neptun radar systems for their night fighters. The Messerschmitt Me 262 jet fighter did not enter service until July 1944, and the lightweight Heinkel He 162 appeared only during the last months of the air war in Europe. The Luftwaffe could not deal with Britain's increasingly lethal defensive fighter screen after the Battle of Britain, or the faster P-51 Mustang escort fighters after 1943.

When the Luftwaffe's fuel supply ran dry in 1944 due to the oil campaign of World War II, it was reduced to anti-aircraft flak roles, and many of its men were sent to infantry units. By 1944 it operated 39,000 flak batteries staffed with a million people in uniform, both men and women.

The Luftwaffe lacked the bomber forces for strategic bombing, because it did not think such bombing was worthwhile, especially following the 3 June 1936, death of General Walther Wever, the prime proponent of a strategic bomber force for the Luftwaffe. They did attempt some strategic bombing in the east with the problematic Heinkel He 177A. Their one success was destroying an airbase at Poltava Air Base, Ukraine during the Allied Operation Frantic, which housed 43 new B-17 bombers and a million tons of aviation fuel.Richard J. Evans, The Third Reich at War (2009) 436–7

Introduction of turbojet-powered combat aircraft, mostly with the Messerschmitt Me 262 twin-jet fighter, the Heinkel He 162 light jet fighter and the Arado Ar 234 reconnaissance-bomber was pioneered by the Luftwaffe, but the delayed period (1944–45) of their introduction – much of which was due to the lengthy development time for both the BMW 003 and Junkers Jumo 004 jet engine designs—as well as the failure to produce usable examples of their two long-developed higher-power aviation engines, the Junkers Jumo 222 multibank 24-cylinder piston engine of some 2,500 hp, and the advanced Heinkel HeS 011 turbojet of nearly 2,800 lb. of thrust, each of which were meant to power many advanced German airframe design proposals in the last years of the war—meant that they were introduced "too little, too late", as so many other advanced German aircraft designs (and indeed, many other German military weapon systems) had been during the later war years.

Although Germany's allies, especially Italy and Finland, had air forces of their own, there was very little coordination with them. Not until very late in the war did Germany share its aircraft and alternative fuel blueprints and technology with its ally Japan, resulting in the Nakajima Kikka jet fighter and the Mitsubishi Shusui rocket fighter, respectively based on the Me 262A and Me 163B—both of which, similarly, came far too late for Japan to improve its defensive aircraft systems, or to make alternative fuels and lubricants.James S. Corum, "The Luftwaffe and Its Allied Air Forces in World War II: Parallel War and the Failure of Strategic and Economic Cooperation," Air Power History Volume: 51#2 (2004).

Britain: Royal Air Force

The British had their own very well-developed theory of strategic bombing, and built the long-range bombers to implement it.Tami Davis Biddle, "British and American Approaches to Strategic Bombing: Their Origins and Implementation in the World War II Combined Bomber Offensive," Journal of Strategic Studies, March 1995, Vol. 18 Issue 1, pp 91–144

Once it became clear that Germany was a threat, the RAF started on a large expansion, with many airfields being set up and the number of squadrons increased. From 42 squadrons with 800 aircraft in 1934, the RAF had reached 157 squadrons and 3,700 aircraft by 1939.Expansion at Last" They combined the newly developed radar with communications centres to direct their fighter defences. Their medium bombers were capable of reaching the German industrial centre of the Ruhr, and larger bombers were under development.

The RAF underwent rapid expansion following the outbreak of war against Germany in 1939. This included the training in other Commonwealth nations (particularly Canada) of half of British and Commonwealth aircrews, some 167,000 men in all. It was the second largest in Europe. The RAF also integrated Polish and other airmen who had escaped from Hitler's Europe. In Europe, the RAF was in operational control of Commonwealth aircrews and Commonwealth squadrons although these retained some degree of independence (such as the formation of No. 6 Group RCAF to put Canadian squadrons together in a nationally identifiable unit).

The RAF had three major combat commands based in the United Kingdom: RAF Fighter Command charged with defence of the UK, RAF Bomber Command (formed 1936) which operated the bombers that would be offensive against the enemy, and RAF Coastal Command which was to protect Allied shipping and attack enemy shipping. The Royal Navy's Fleet Air Arm operated land-based fighters in defence of naval establishments and carrier-based aircraft. Later in the war the RAF's fighter force was divided into two Air Defence of Great Britain (ADGB) for protecting the UK and the Second Tactical Air Force for ground offensive support in the North West Europe campaign.

Bomber Command participated in two areas of attack – the strategic bombing campaign against German war production, and the less well known mining of coastal waters off Germany (known as Gardening) to contain its naval operations and prevent the U-boats from freely operating against Allied shipping. In order to attack German industry by night the RAF developed navigational aids, tactics to overwhelm the German defences control system, tactics directly against German night-fighter forces, target marking techniques, many electronic aids in defence and attack, and supporting electronic warfare aircraft. The production of heavy aircraft competed with resources for the Army and the Navy, and it was a source of disagreement as to whether the effort could be more profitably expended elsewhere.

Increasingly heavy losses during the latter part of 1943 due to the reorganized Luftwaffe night fighter system (Wilde Sau tactics), and Sir Arthur Harris' costly attempts to destroy Berlin in the winter of 1943/44, led to serious doubts as to whether Bomber Command was being used to its fullest potential. In early 1944 the UK air arm was put under Eisenhower's direct control where it played a vital role in preparing the way for the Overlord Invasion.Richard J. Overy, The Air War, 1939–1945 (1981)Tami Davis Biddle, "Bombing By The Square Yard: Sir Arthur Harris At War, 1942–1945," International History Review, vol 9#1 1999, pp 626–664

Soviet Union: Soviet Air Force

By the end of the war, Soviet annual aircraft production had risen sharply with annual Soviet production peaking at 40,000 aircraft in 1944. Some 157,000 aircraft were produced, of which 126,000 were combat types for the Voyenno-Vozdushnye Sily (Военно Воздушные Силы, lit. "Military Aerial Forces") or VVS, while the others were transports, trainers, and other auxiliary aircraft.R.J. Overy, The Air War: 1939–1945 (1980) p 150 The critical importance of the ground attack role in defending the Soviet Union from the Axis' Operation Barbarossa through to the final defeat of Nazi Germany with the Battle of Berlin resulted in the Soviet military aviation industry creating more examples of the Ilyushin Il-2 Shturmovik during the war than any other military aircraft design in aviation history, with just over 36,000 examples produced.

During the war the Soviets employed 7500 bombers to drop 30 million bombs on German targets, with a density that sometimes reached 100–150 tons/ sq kilometer.A. A. Sidorenko, The Offensive (Moscow, 1970; USAF translation) p. 8James Sterrett, Soviet Air Force theory, 1918–1945 (2007) pp 86–131

United States: Army Air Forces

Before the attack on Pearl Harbor and during the period within which the predecessor U.S. Army Air Corps became the Army Air Forces in late June 1941, President Franklin D. Roosevelt gave command of the Navy to an aviator, Admiral Ernest King, with a mandate for an aviation-oriented war in the Pacific. FDR allowed King to build up land-based naval and Marine aviation, and seize control of the long-range bombers used in antisubmarine patrols in the Atlantic. Roosevelt basically agreed with Robert A. Lovett, the civilian Assistant Secretary of War for Air, who argued, "While I don't go so far as to claim that air power alone will win the war, I do claim the war will not be won without it."