I seem to remember him saying he would;
Here is one of them;
"In April 2011, Trump said that when President Barack Obama produced "his birth certificate ... I'd love to give my tax returns." Obama's birth certificate was released a week later, resulting in Trump's saying his tax returns would be released "at the appropriate time"
also
!In 2012, Trump sought to have Republican presidential candidate Mitt Romney's tax returns released on April 1, which "historically is the time that everybody gives them". That year, Trump also said that not seeing a presidential candidate's tax returns would lead people to think there was "almost, like, something wrong. What's wrong?"
Etc;