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At some point, it would have been a blank box (apparently the natural state of a TARDIS), but thanks to the Chameleon Circuit (which apparently every TARDIS has) it changed its appearance to British Police Box in the first episode, to blend in to its surroundings... Then it got stuck on Police Box, due to the Chameleon Circuit apparently being broken. When (in the 2nd episode) Susan realised that it had become stuck on Police Box, she told Ian & Barbara some of the other forms the TARDIS had taken. These included a sedan chair & a Greek column but not a phone booth.

the Doctor has never fixed the Chameleon Circuit, partly I think because he's just gotten used to it that way, and I would wager that the TARDIS herself maybe likes it too, as she repaired herself all nice and neat in "The Eleventh Hour", including an update to the Police Box appearance... But still looks like a police box! In the first "Meanwhile in the TARDIS" scene (which follows "The Eleventh Hour"), the Eleventh Doctor told Amy that the TARDIS, immediately on landing, does still analyse her surroundings & devise a suitable appearance -- but then always makes herself look like a Police Box from 1963, anyway.

In "Attack of the Cybermen", the Sixth Doctor attempted to fix the Chameleon Circuit and did manage to get the TARDIS to change her appearance -- but never to anything that was actually appropriate to her surroundings. He eventually got very annoyed with it not really working & settled for accepting the constant Police Box "disguise".

See youtube, if you search 6th doctor it's on the first page.

the Doctor Who Magazine comic strip "Hunters of the Burning Stone" reveals that the Eleventh Doctor was responsible for the TARDIS being locked in its police box shape.