German scientists began experimenting with guided bombs before the outbreak of the Second World War. The Fritz-X bomb was used for the first time in September 1943. The 1400kg armour-piercing bomb had small wings and a tail unit containing the guidance mechanism. It was controlled during its drop by an observer in the aircraft passing commands via a radio link. It had a range of eight miles and was so fast that anti-aircraft guns found it impossible to hit them. As well as sinking the battleship Roma it also badly damaged the British warship Warspite in 1943.

The United States also developed the VB-1 guided bomb in the later stages of the war. The 1,000lb missile had a pair of radio-controlled rudders, which were manipulated by a bombardier in the aircraft after it was dropped. a flare in the bomb's tail was used to provide visual guidance. An improved VB-3 could also have its range adjusted by altering the angle of its descent.