The Prime Minister appeared to blurt out the plan to raise the threshold at which workers start paying NI from £8,628 a year to £12,000, during a campaign visit to Teesside.
Speaking during a question-and-answer session with staff at an engineering plant in Middlesbrough, he said the Conservatives were committed to a policy of "low tax for... working people".
He went on: "If you look at what we're doing and what I've said in the last few days, we are going to be cutting National Insurance up to £12,000, we are going to be making sure that we cut business rates for small businesses.
"We are cutting tax for working people."
Mr Johnson originally raised the idea of upping the threshold for NI contributions during the Conservative leadership contest last summer - although at that stage he did not put a figure on it.
The Institute for Fiscal Studies (IFS) calculated that for each £1,000 that it was raised, 600,000 workers would be lifted out of NI altogether.
If just employee and self-employed thresholds were lifted the cost to the Exchequer would be £3 billion a year rising to £4.5 billion if the employer threshold was raised as well.
To raise the threshold to £12,500 - as Mr Johnson's then leadership rival Dominic Raab was proposing - was put at £11 billion, rising to £17 billion if the employer threshold was included.
The IFS said that 2.4 million workers would be taken out of NI.