Jose Mourinho said in the week it would be a disgrace if his star-studded side lost to the League One outfit, but even he was quick to congratulate Phil Parkinson’s Bradford after a simply stunning victory.
It looked like being a comfortable afternoon for Chelsea after Gary Cahill flicked in a corner and Ramires then doubled the advantage.
But Jonathan Stead gave the visitors a lifeline before the break when he drilled a fine strike past Petr Cech.
That proved pivotal as the Bantams came out with renewed confidence in the second half and soon levelled through Felipe Morais.
The goal sent the 6,000 travelling fans crazy but it was to get even better as Andrew Halliday put them ahead after 81 minutes.
Mourinho threw on Cesc Fabregas, Willian and Eden Hazard in a desperate bid to rescue a draw but Mark Yeates secured a sensational win with a fourth goal for Bradford in injury-time.
It was no more than the Bantams deserved, either, after more than matching Chelsea for most of an enthralling encounter.
Even though the Blues made nine changes from their Capital One Cup semi-final, their starting line-up was still estimated to have cost around £98m, compared to Bradford’s £7,500.
Petr Cech returned in goal for Chelsea after missing out in midweek and was called upon to make a superb early stop from Andrew Davies’ header.
Bradford were threatening at set pieces, but it was Chelsea who took the lead from a corner when Cahill flicked in from Oscar’s delivery.
Didier Drogba then forced Ben Williams into a good save with a curling shot before Ramires played a one-two with Mohamed Salah and executed a neat finish to double the lead.
Bradford might have buckled, but former Premier League striker Stead netted with a thunderous shot from the left edge of Chelsea’s box minutes before half-time.
The goal kept the visitors in contention and they started the second half on the front foot as James Hanson forced Cech to save a header and Billy Knott curled an effort over.
With Chelsea continuing to struggle, Mourinho threw on Willian and Fabregas, but even they could not stop the Bantams equalising.
James Meredith’s long throw was flicked on by Hanson, Knott forced Cech to save and the ball fell to Morais to finish into the empty net.
It was to get even better for the travelling fans as Stead’s lay-off was dispatched by Halliday to put Bradford ahead.
Chelsea’s best chance to equalise fell to Zouma but he shot over the bar and Yeates then secured a famous win when he netted the fourth in injury-time.