Prototype TARDIS

Prototype TARDISes, (AUDIO: Collision Course, PROSE: Cold Fusion) or Proto-TARDISes as Alice Obiefune called them, (COMIC: The Lost Dimension) referred to as proto-timeships in The Book of the War, (PROSE: The Book of the War) were early TARDIS designs created under the rule of Rassilon in early Gallifreyan history. (PROSE: Heart of TARDIS, COMIC: The Lost Dimension) Some prototypes were retroactively numbered Type 1s. (PROSE: Heart of TARDIS)

Several malfunctioning prototype TARDISes instigated Multi-Doctor Events when they threatened to destroy reality. (PROSE: Cold Fusion, Heart of TARDIS, COMIC: The Lost Dimension, AUDIO: Collision Course)

History
Even before the anchoring of the thread which created the modern structure of time, "proto-timeships" of the Great Houses began embedding themselves at important points of the still-fluctuating future. On the day of the anchoring of the thread, field agents took their places inside these timeships while the heads of the six Ruling Houses gathered at the machine-heart on the Homeworld. While the site of the machine-heart became the centre of history, the positions of the proto-timeships became the "strategic points" holding the various threads of history in place and connecting them to the central node. (PROSE: The Book of the War)

The first piloted TARDIS flight occurred on Henlen and involved a prototype TARDIS with six pilots including Tompino, Punshon, and Ankarrie. This flight was sabotaged by the Sirens of Time, who caused it to explode. The explosion, other than killing the pilots, produced a massive temporal paradox, who split reality apart: different timelines converged, time holes between different ages were opened, and Henlen became a planet devoid of identity, always changing. In two different timelines, the Fourth Doctor landed on Henlen, once accompanied by Leela and once by Romana, and discovered the truth. This caused Leela and Romana, in the main timeline, to remember those events, and travelled to Henlen to stop the experiment from happening. They succeeded, but as a result, they prevented Time Lords from ever discovering time travel. Six incarnations of the Doctor (from the third to the eighth) were then summoned to the planet by Bernice Summerfield, and together with Romana they brought to the experiment to a successful conclusion, thus ensuring the correct timeline would happen. (AUDIO: Collision Course)

The Fifth Doctor believed the Machine to be a prototype TARDIS. (PROSE: Cold Fusion)