In the majority of cases bombs were aimed visually, by aircraft flying horizontally from a great height. Two basic type of bombsight were used.
The Vector Sight
The bombardier had to set the aircraft speed and altitude, determine wind speed, direction and the bomb load ballistic data. He would look at a sighting cross made of thin pieces of wire, or lines of light on a screen, which showed where the bombs would hit if dropped at that particular moment. The Vector Sight needed a long horizontal approach and was not suited to a monoplane which tended so shift sideways as it banked for turns. In 1942, the RAF produced the Mark 4 which was gyro-stabilized and allowd the aircraft to make banked turns on a bomb run. It worked by passing the data to a complex analouge computer which corrected the sight to take account of any course deviantion.
During a bomb run a bombardier looked through a sighting telescope at the target. The sight was adjusted by a variable speed eöectric motor. The bombardier input bomb load and altutude data, then set the telescope sight over the target. The base of the sight was gyrostabilized and, by keeping the telescope on the target, aircraft movement information was fed into the sighting computer. The computer produced course correction signals that were channeled to the pilot´s panel. When the bomber was close to the target, the sight telescope was almost vertical and the computer calculated when the release angel had been reached. Then a series of electric contacts were closed and the bombs dropped automaticlly.
The US version of the Tachometric sight was called "NORDEN"¹. Later models of the "Norden" channeled data directly into the auto pilot and effectively, the bomb aimer could "fly" the plane by fine adjustment of the sighting telescope. The main disadvantage of this sight was that it needed at least 20 seconds of nondeviational flight. It also lost accuracy if fire and smoke obsured the target.
¹ Also called "Norden"