Goals from James Hanson, Rory McArdle and leading scorer Nahki Wells settled the contest inside half an hour and made it a happy return to the capital for Phil Parkinson's side and 25,000 of their fans, three months after their historic Capital One Cup final appearance.
The Bantams, last promoted when they reached the top flight in 1999, have just completed their sixth season in the bottom tier, but clearly benefited from their chastening Wembley experience at the hands of Swansea in February and swept aside Northampton, who failed to get to grips with the occasion.
Bradford settled quicker and after hogging possession were rewarded for their early composure with the game's opening goal in the 15th minute.
Northampton failed to clear Kyel Reid's cross adequately and when the ball broke on the right edge of the penalty area, Garry Thompson picked out Hanson at the far post and the big striker guided his header over stranded Cobblers goalkeeper Lee Nicholls into the net.
Northampton had hardly got a foothold in the game when the West Yorkshire club doubled their lead four minutes later. This time Nathan Doyle's fine first-time cross to the near post was met by McArdle, who stole in ahead of a flat-footed defence to head powerfully home.
Aidy Boothroyd's boys were being over-run by a City side playing like they had been here before and when Wells headed home their third goal in the 28th minute, the contest appeared over.
Thompson again played a key role, heading the ball back across goal following Reid's cross for an unmarked Wells to volley home his 26th goal of the season and his eighth in as many games.
There was no let-up from Bradford after the restart and only a finger-tip save from Nicholls denied the Bantams a fourth goal after Carlisle's attempted clearance had rebounded goalwards off Hanson. City fans paid tribute to the 56 fans who lost their lives at Valley Parade in 1985 with cheers and applause in the 56th minute, one week after the fire disaster's 28th anniversary.
Adebayo Akinfenwa's tame header at the end of a rare foray forward summed up the Cobblers' disappointment and City cruised through the closing stages to spark joyous scenes of celebration among their fans.
Source: PA
Source: PA