Donald Trump's personal physician said he wrote a letter declaring the Republican nominee would be the healthiest president in history in just five minutes while a limo sent by the candidate waited outside his Manhattan office.

Dr. Harold Bornstein, who has been Trump's doctor for 35 years, told NBC News on Friday that he stands by his glowing assessment of the 70-year-old's physical state.

"His health is excellent, especially his mental health," he said in an exclusive interview at his Park Avenue office.

In his letter, Bornstein said there were no "significant medical problems" in Trump's history and that a recent examination "showed only positive results." The letter was written and released in December but has drawn fresh scrutiny in recent days as Trump's allies have questioned Hillary Clinton's health.

Read More