The USAAF weren't forced into daytime bombing, they determinedly pursued the policy in the face of British scepticism.

Both the RAF and USAAF believed that tight formations of bombers with powered gun turrets would be able to defend themselves against enemy fighters. The RAF rapidly discovered this was not the case, suffering heavy losses in 1939 raids such as the Battle of Heligoland Bight where 24 Wellington bombers were sent against the German fleet and only half returned for no result. From 1940 the vast majority of Bomber Command's missions were flown at night, reducing losses, but also accuracy.

The USAAF's Air War Plans Division drew up a plan based on targeting key nodes of the German economy including electric power, transportation, oil and aircraft production. Accurate targeting was only possible in daylight; the US had faith in the B-17, carrying more and heavier guns than British bombers. Churchill and Sir Charles Portal, Chief of the Air Staff, were deeply concerned that the results would be as disastrous as early British efforts and thought it would be wise for them to, at the very least, make preparations for night bombing. Sir John Slessor, Assistant CAS, was more optimistic and believed, almost certainly correctly, that to argue too forcefully would cause annoyance at best, redirection of resources at worst.

At the Casablanca conference in January 1943 Major General Ira C. Eaker, head of US VIII Bomber Command, produced a detailed brief outlining The Case for Day Bombing for the Combined Chiefs of Staff and a shorter synopsis that he personally presented to Churchill. Though still sceptical Churchill saw the potential benefits of round-the-clock operations and the result was the Casablanca Directive, a rather vague proclamation that allowed each air force to pursue its preferred policy.

The USAAF were no more successful with unescorted bomber raids, suffering heavy losses as they pushed into Germany in 1943 particularly over Schweinfurt, but the introduction of long-range escort fighters allowed them to break the back of the Luftwaffe in early 1944 and maintain daylight operations.

Tami Davis Biddle's article "British and American Approaches to Strategic Bombing: Their Origins and Implementation in the World War II Combined Bomber Offensive" in the Journal of Strategic Studies gives more details of how the strategies were developed and evolved.