Bomber Command is an organizational military unit, generally subordinate to the air force of a country. Many countries have a "Bomber Command", although the most famous ones were in the United States and Britain. A bomber command is generally composed of bombers (i.e. planes used to bomb targets).
The RAF Bomber Command was most famous during World War II, when its aircraft were used for devastating night time air raids on Germany and occupied Europe. At its height, Bomber Command under Air Chief Marshal Sir Arthur Harris could put over 1,000 aircraft into the air over Germany. Over 12,000 Bomber Command aircraft were shot down during World War II, and 55,000 aircrew were killed. Various aircraft were used, from the obsolete and horrendously vulnerable Fairey Battle in 1939 to the command's most numerous and successful aircraft, the Avro Lancaster. Not only British aircraft were used. American machines such the B-17 Flying Fortress) saw service as well.
Whereas the Bomber Command in the RAF was a single organisation, reporting directly to the Chief of the Air Staff, there were many American Bomber Commands. They were subordinate formations, reporting in general to various numbered Air Forces around the world. Out of all those organisations, two stand out; VIII Bomber Command and XXI Bomber Command.
VIII Bomber Command was the UK-based (there were also substantial strategic air forces in Italy) American part of the strategic bomber offensive, Operation Pointblank. Two aircraft made up the backbone of this unit, the B-17 Flying Fortress, and the B-24 Liberator. The former was more famous, but the latter was longer ranged and had a larger bomb load. The USAAF came to Europe wanting to bomb by day with 'precision'. True precision bombing in the modern sense was impossible in the 1940s. However, daylight bombing was more accurate than night bombing. The big problem was that bomber in daylight were considerably easier to shoot down. The RAF had tried day bombing early in the war, but had abandoned it in the face of huge losses. However, the USAAF persevered. It was not until long range escort fighters like the P-47 Thunderbolt and the P-51 Mustang came into service that daylight bombing really worked. The original American doctrine of heavily armed bombers defending themselves over enemy territory was found to be fundamentally flawed.
In the Pacific, XXI Bomber Command was the main instrument of destruction used against Japan itself. Its B-29 Superfortresses, operating from the Marianas, were the longest range and most modern bomber in service in the world at the time. Again, as in Europe, the USAAF tried daylight precision bombing. However, it proved to be impossible due to the weather around Japan, as bombs dropped from great height were tossed about by high winds. LeMay, commander of XXI Bomber Command instead switched to mass firebombing attacks by night from low level. Japanese cities were uniquely vulnerable to this sort of attack, being closely packed and largely built of wood.
The Pacific attacks included the most devastating air raid in histry. It was not, as some might think the result of dropping one of the two atomic bombs. It was a conventional raid on Tokyo which created a firestorm and killed 100,000 people. However, the atomic bomb attacks on Hiroshima and Nagasaki which helped to end the war were devastating enough, killing tens of thousands of people.
The Bomber Commands of the RAF and USAAF were responsible for most of the civilian casualties that the UK and US inflicted upon the enemy in World War II. There are those who condemn the attacks as against international law. It should be remembered that it was the Luftwaffe that first attacked cities with massed bomber formations in that war. It should also be remembered that World War II was a total war, the like of which has not been seen since on a worldwide scale. In total war, many constraints that would normally bind the behaviour of the combatants are often loosened.