Type 40

The Type 40 — or, formally, the Type 40 time travel capsule — was the class of TARDIS to which the Doctor's TARDIS belonged. (TV: Victory of the Daleks, The Pirate Planet, State of Decay and others)

Information
Little was actually known about the type, aside from the example provided by the Doctor's TARDIS herself. However, because of the Doctor's penchant for changing the configuration of his machine — he impulsively added a found tribophysical waveform macro-kinetic extrapolator for additional shielding. (TV: Boom Town)

Obsolete
The one quality on which most of the Doctor's Time Lord contemporaries agreed was that Type 40s were antiques. At one time or other, the Monk, the Master, and the Rani all described the Doctor's TARDIS as undesirably old. (TV: The Time Meddler, The Claws of Axos, Time and the Rani) Romana, likely the youngest of the Doctor's contemporaries, indicated that the type was no longer on the syllabus at the Time Lord Academy. (TV: The Pirate Planet) Indeed, once when the Fourth Doctor returned to Gallifrey, Commander Hilred called the type "obsolete" and noted that all had been pulled from service. (TV: The Deadly Assassin) Professor Chronotis, who was clearly of an older generation than the Doctor, claimed that Type 40s first came out when he was a boy. (TV: Shada / WC: Shada / AUDIO: Shada) When the TARDIS inhabited Idris, it mentioned to the Doctor that it was already a museum piece when the Doctor was young. (TV: The Doctor's Wife) It seems, however, that while they were decommissioned, they were not all destroyed, as another was activated by Straxus to stand in for the Doctor's (AUDIO: Dark Eyes).

Other distinctive qualities
Exactly 305 were said to have been grown and officially registered with the Gallifreyan authorities. (TV: The Deadly Assassin) All were installed with the Record of Rassilon. According to Romana, only the Type 40s were so equipped. (TV: State of Decay)

Behind the scenes

 * The Doctor's TARDIS was not always referred to as a "Type 40". Initially, there was no suggestion that the TARDIS was anything other than a unique craft. However, with the broadcast of The Time Meddler, the notion was seeded that, like cars and other devices, TARDISes were produced with mark numbers. The Doctor's TARDIS was then called a Mark I, while the Monk's was a superior Mark IV. Many fans have retconned this early Hartnell model number with the later "Type 40" by suggesting that the Doctor's TARDIS was a "Type 40, Mark I", or "Mark I, Type 40". There is, however, no dialogue which calls for such an assumption. The actual type numbers of other Time Lords' TARDISes has never been given in televised stories. However, other type numbers have been mentioned in other media. More recently, the Doctor's TARDIS has been given the designation TT Type 40, Mark III by the onboard computer on the Teselecta in the episode  Let's Kill Hitler