Page "TARDIS" Paragraph 8

When Doctor Who was being developed in 1963, the production staff discussed what the Doctor's time machine would look like.

To keep the design within budget, it was decided to make it resemble a police box.

This was explained in the context of the series as a disguise created by the ship's " chameleon circuit ", a mechanism which is responsible for changing the outside appearance of the ship in order to fit in with its environment.

The Ninth Doctor explains that if, for example, a TARDIS ( with a working chameleon circuit ) were to materialise in ancient Rome it might disguise itself as a statue on a plinth.

The First Doctor explained that if it were to land in the middle of the Indian Mutiny, it might take on the appearance of a howdah ( the carrier on the back of an elephant ).

A further premise was that the circuit was broken, explaining why it was " stuck " in that form.