American air force policy favored 'precision' bombing, particularly before 1944. This policy had little sway, however, given the British head start in bombing. British bombers carried heavier payloads than American planes, and were in the field three years before the Americans arrived. Given the immense industrial potential of the German economy, RAF policy was to protect its planes at all costs. This had caused them to withold fighters from the French, and would lead them to chose night-bombing rather than the more accurate daylight raids favored by the Americans. Even so, their losses were substantial, at times higher than those of the Germans on the ground. As early as 1940, the problem of finding primary targets was such that a large fraction of British bombers simply dumped their loads on the nearest marshalling yards or other targets. In 1942, Sir Arthur Harris was appointed to head the RAF bomber command. He had noted, correctly, the low efficacy of the bombers in taking out military targets, but thought that some success might be had in bombing civilian areas, lowering morale and keeping workers away from factories. He was able to convince others of this scheme, largely through exaggerating the success of the RAF.
The Americans arrived in force in 1943, and preferred more precise attacks where possible. Of course, the precision of any heavy bomber is open to question. In the words of one wing commander in the Gulf war, "The B-52 is a high-precision weapon. You drop a load from 20,000 feet and I guarantee you it will hit the ground every time." In January of 1943, the Casablanca conference outlined the goals of strategic bombing, describing both industrial- and morale-destruction as aims. However, in the enumeration of targets, only military and industrial targets were listed. By that summer, air-engine plants and ball-bearing plants became preferred targets. It was discovered somewhat after the fact that these relatively more concentrated attacks were effective. Albert Speer reported after the end of the war that ball-bearing shortages almost brought Germany to its knees. At the time, this success was not apparent.
When the planning for the Normandy invasion began, transportation targets became a high priority. By nature, these are largely civilan targets. Switchyards are in cities, and the lines are used far more by civilians than by the military. This attack had seven phases, and the USAAF listed the goal of the fifth phase as general paralyzation.
After bases in Western Europe were established, large raids became common, and the Germans (under Speer’s direction) responded by decentralizing industry. At this point, allied air dominance made the destruction of entire cities possible, and not much an extension of previous plans. Targetting was difficult and area raids became the norm. The fire-bombing of Dresden, Hamburg and other towns followed.
On the Pacific front, area bombing was common from the start, in part because of racist considerations, in part because the cities were denser and more flammable, in part from fury over the bombing of Pearl Harbor. It should be remembered that the atomic bombings were remarkable only for the technology involved, not the destruction of Hiroshima and Nagasaki. Other cities had suffered far worse.
from my homework for Historical Studies B-54 on October 28, 1991.