With the advent of aircraft being able to drop bombs on enemy targets in World War I, every country with a burgeoning fleet of bombers struggled with dropping bombs on target. By World War II, the combatants resorted to sending hundreds (and later in the war, thousands) of bomber aircraft over a target with the hope that a few bombs would score a lucky hit on the intended target. It was common for bombs to miss the intended target by miles! The problem was so pervasive, that British Bomber Command's strategy throughout the war was to simply target entire cities, the logic being that even bombs that miss by miles would surely still hit somewhere in the city. British Bomber Command measured its success essentially on how many German cities that it leveled. Such was British strategy that Bomber Command focused its efforts on nighttime bombing raids since the pilots and bombardiers didn't really need to see their target - the target city was below them somewhere, their exact position over the city was quite irrelevant.
The United States Army Air Corps (it was not yet called the United States Air Force) took a different approach. The U.S. believed that it could pick specific targets for ariel bombardment, such as factories, rail yards, refineries, etc. In theory, such a strategy has no downsides since fewer bombs means less men being put at risk, less material being expended and less collateral damage. But how did the U.S. intend to accomplish this strategy when bombs were missing by miles? It doesn't take much of an imagination to understand how difficult it is to drop a bomb from an aircraft moving at several hundred miles per hour, from an altitude of several thousand feet, through a turbulent atmosphere on to a target that more often than not is protected by anti-aircraft guns and pumping thousands of exploding shells into the path of the bombers.