Unlike other PVRs the box - codenamed "Pandora" by the BBC and being developed by tech company Promise TV - does not need to be programmed to record specific shows.
Instead, it simply stores every Freeview programme broadcast, using a massive hard drive. Viewers can then effectively fast forward and rewind across a whole week's output, as well as searching by genre.
Work on the prototype is being funded by BBC research and development. The division has poured tens of thousands of pounds into the project, but insists it is not interested in developing hardware, and is only investing in Pandora to experiment on how viewers navigate through programmes.
The corporation wants to make sure that, as PVRs become commonplace, viewers will still be able to find its shows.
The storage capacity of the box caused ripples throughout the PVR industry when it was unveiled at the Open Tech conference in London on 23 July.
"This could well herald a change in the way we watch television," said Promise TV's Dominic Ludlam. "The freedom from advance programming means that TV programmes need never be missed, even if they are only heard about after they've aired."