Back in 2011, Trump sat down with Kelly to promote a Republican presidential debate the real-estate mogul was planning to moderate.
Trump criticized several of the candidates who wouldn't commit to the event, which was to be hosted by the conservative news organization Newsmax.
"We're not seeing a lot of courage here, are we? Not lots of courage. These Republicans, they're supposed to be brave," Trump told Kelly at the time.
He also said he "could never" be a better moderator than Kelly, who he said was doing "a great job, and I mean it."
According to Newsmax, Trump ultimately backed out of moderating the organization's debate, as he was considering a potential 2012 presidential campaign of his own.
Trump, now the Republican presidential front-runner, has launched raging attacks against Kelly since last August, when she was one of three moderators at the first Fox-hosted debate. At the time, Kelly infuriated Trump with a pointed question about disparaging things he's said about women's looks.
The billionaire repeatedly complained that Kelly would treat him fairly at Fox's upcoming debate on Thursday. He even polled his Twitter followers asking if he should attend the debate, given Kelly's alleged bias against him.
After Fox mocked Trump's Twitter poll with a sarcastic statement, Trump announced Tuesday that he would boycott the debate.