To the delight of their 6,000 travelling army of supporters, the Bantams came from two goals down to dump Jose Mourinho’s men out of the Cup.
Goals from Jon Stead, Filipe Morais, Andy Halliday and Mark Yeates sent Bradford through on an afternoon that no supporter of the Valley Parade club will ever forget.
Phil Parkinson’s men were simply magnificent throughout to make a mockery of the huge gulf between the two clubs.
Stead was man of the match with a stunning goal that dragged the League One outfit back into the tie just before half-time that he then followed with vital assists to set-up Halliday and Yeates.
But the former Huddersfield Town striker was far from the only hero for Bradford as they pulled off one of the all-time Cup shocks in magnificent fashion.
City started in a manner that clearly demonstrated they hadn’t - as Mourinho often puts it when criticising negative visitors “come to park the bus”.
With Billy Knott playing through the centre up front and flanked by James Hanson and Jon Stead, the Bantams clearly hadn’t come to west London intent on merely making up the numbers.
Such a bold approach almost brought the afternoon’s first goal, as Andrew Davies powered a header goalwards from a Filipe Morais corner that Petr Cech did brilliantly to keep out.
Coming just moments after Gary Liddle’s 20-yard shot had been blocked bravely by Drogba, Chelsea knew they were in a game.
As all great sides do, however, once that became apparent the hosts merely stepped up a gear and went ahead on 21 minutes.
Gary Cahill’s near post flick with the outside of his right foot was great improvisation by the England defender. Nevertheless, City should never have allowed Oscar’s corner to get to Cahill and it was a poor goal to concede.
After that, the Blues poured forward and Ben Williams had tio be at his best to keep out a Drogba shot before Mohamed Salah and Loic Remy got in each other’s way to hand Bradford a major let-off as the ball trickled through to the goalkeeper.
Chelsea were much more clinical seven minutes from time when Ramires, after first dispossessing Morais and then exchanging passes with Mohamed Salah, fired in off the post.
At that stage, the afternoon could have gone badly wrong for the Bantams.
Instead, Parkinson’s men responded with a goal of their own as Stead collected a pass on the edge of the area before hitting a fierce left foot shot that Cech got a hand to but could not keep out.
Scoring just before the break gave City plenty of momentum after the restart as they forced four corners in the opening ten minutes, much to the delight of their noisy 6,000 strong travelling army of fans.
Only one, however, came close to bringing a goal as Hanson headed into the hands of Cech. Bradford carried more of a threat in open play as, first, Morais had a shot tipped over by the Blues goalkeeper and then Billy Knott curled an effort wide from the edge of the area.
At the other end, Oscar had a great opportunity following a lovely back-heel by Cesc Fabregas but dragged his shot wide and Bradford made the Brazilian pay 15 minutes from time when Morais tapped in after Knott’s shot had been blocked.
City weren’t finished there, however, as the League One side took the lead on 82 minutes when Andy Halliday fired past Cech to send the Bantams fans into delirium.
It wasn’t over then, Yeates coming off the bench to fire in after being set up by Stead with another fine pass to send Bradford through to round five.
It was a memorable day for the Bantams, who beat Arsenal and Aston Villa en route to the 2013 League Cup final, where they lost to Swansea.
“In terms of the performance, the magnitude of beating Chelsea today, that’s got to beat them all,” Parkinson added.
“It’s only slightly sinking in. When we were on the pitch celebrating at the end it did feel a bit surreal, that we’ve come to the league leaders and scored four goals.
“It’s a great feeling and will be remembered in Bradford for a long time, but the rest of the country as well.”
At 2-0 down, to goals from Gary Cahill and Ramires, many expected Bradford to buckle, but not Parkinson.
“(We) just wanted half-time to come,” he said.
“To get the goal back, and it was a terrific strike from Jon, gave us a real lift and belief.
“Kicking towards our fans in the second half we always felt that we could cause them problems and we certainly did.”