The number of front-line combat aircraft operated by the air arms participating in World War II grew from 6,000 in September 1939 to peak of 44,000 at the end of 1944. Part of this increase came as more countries entered the war - in September 1939 the only major combatants were Germany, Britain, France and Poland, while Italy entered the war in 1940 and the USSR, the US and Japan in 1941. Most combatants also grew their air arms during the war, with the greatest increase being that of the US: the combined front-line strength of the USAAF and US Navy grew eight-fold from 2,200 in December 1941 to over 17,500 at the end of 1944.

The most distinctive feature of the data is the relative success of the Allies in increasing their front-line strength. The two sides remained broadly in balance for the first two years of the war, and this situation was maintained even after the start of the Pacific war. But from 1942 the Allies grew their front-line strength much faster than the Axis, and by December 1944 had nearly four times as many aircraft as the Axis.

Comparing totals of front-line combat aircraft between countries is complicated by the fact that different air arms are organized in different ways. Published totals for front-line combat aircraft usually include reserve or training aircraft for some countries but not for others. To provide an "apples to apples" comparison, we define front-line combat aircraft to be all combat aircraft, other than reserve aircraft, held by combat units. Despite the term “front-line”, geography is not a factor in the definition, since the air war has no front line in the traditional sense and even locations remote from combat needed defense against enemy aircraft and shipping. Serviceability is not part of our definition: depending on the air arm and stage of war, the number of front-line aircraft servicable varied between 50% and close to 100% of the number we give.

Combat aircraft are those whose primary purpose is combat, i.e. deploying weapons such as guns, bombs or missiles against the enemy. The major categories of combat aircraft are fighters and bombers. Gun-carrying ground attack aircraft and maritime patrol aircraft that carry anti-submarine or anti-shipping weapons are also considered combat aircraft. Reconnaissance and transport aircraft are excluded from our definition whether or not they carry defensive armament. Combat units are squadrons, groups or wings whose primary purpose is combat. Dual role units such as fighter-reconnaissance, bomber-reconnaissance and bomber-transport units are included as combat units. Units whose primary purpose is operational training are not counted as combat units, even though they may have a secondary combat role - many air arms used their operational training units in combat at critical phases of the war.

Figure 2 below shows the number of front-line combat aircraft for five countries, the US, Great Britain, the USSR, Germany and Japan, and for three country groupings, "Commonwealth". “other Allies” and “other Axis.” Foreign units integrated into the air arms of the major countries are included in the country totals. This factor is most significant for Britain’s RAF which included a large contingent of non-British squadrons from countries including Australia, Canada, France, New Zealand and Poland. "Commonwealth" includes Australian, Canadian, Indian, New Zealand and South African forces not integrated with the RAF. "Other Allies" includes France and Poland in September 1939, China from December 1941, and various small contingents (French, Italian etc.) in the final months of the war. “Other Axis” includes Bulgaria, Finland, Hungary, Italy, Romania amd Vichy France. Air arms involved in separate secondary conflicts (e.g. Finland and the USSR before June 1941 and Japan and China before December 1941) are excluded.