The doomsday bomb was a superweapon kept in Earth's secret weapons store. (COMIC: Cyber-Mole) According to the Eighth Doctor, humans developed the doomsday bombs, which were nuclear, during their "finest hour of paranoia". (AUDIO: To the Death)
A force of Cybermen used the Cyber-Mole to drill beneath London into the Earth's secret weapons store to steal the doomsday bomb. They achieved this and held the world ransom with the bomb. The Second Doctor brought his information on Cybermen to the military and led an assault on the Cyber-Mole using anti-Cybermen weapons, defeating them. (COMIC: Cyber-Mole)
During the 22nd century Dalek invasion reprise, the Monk kept a doomsday bomb among his new art collection as "insurance" against the Daleks. He told his assistant Tamsin Drew that its container was only some files he hadn't unpacked yet. (AUDIO: To the Death)