Strategic bombing during World War II was greater in scale than any wartime attack the world had previously witnessed. The strategic bombing campaigns conducted by Nazi Germany, the United Kingdom, the United States and the Empire of Japan used conventional weapons, incendiaries, and nuclear weapons.
Before the outbreak of the war, Luftwaffe General Walther Wever had advocated creating a strategic bombing capability for the German armed forces, possibly in reference to Hitler's stated desire in Mein Kampf to attack the Soviet Union-General Wever advocated the so-called Ural-Bomber project, which resulted in the design of the four engined Dornier Do 19, and the adaptation of the Junkers Ju 89, also a four-engined aircraft, as strategic bombers. Wever's death in June 1936, however, resulted in the cancellation of the Ural-Bomber concept, with Luftwaffe generals Ernst Udet and Hans Jeschonnek advocating medium bomber production instead, citing that each heavy bomber built would use up the resources to build at least two medium bombers.
The war started on 1 September 1939 with German aerial bombing of civilians in the Polish town of Wielun. From the beginning of the war the Luftwaffe engaged in massive air raids against Polish cities, bombing civilians, hospitals, and refugees. The city of Frampol was bombed by the Luftwaffe and was 90% destroyed as a form of live experiment; a similar fate befell Polish Wieluń.
The president of the United States (then a neutral power), Franklin D. Roosevelt, issued a request to the major belligerents to confine their air raids to military targets. The next day French and the British agreed to abide by the request which included the provision that "upon the understanding that these same rules of warfare will be scrupulously observed by all of their opponents". Berlin waited until the 18 September before agreeing to the request.