A bullet whizzing just past Donald Trump's head was caught in a photo during Saturday's assassination attempt at a Pennsylvania campaign rally ... and it barely missed him.

NYT photog Doug Mills took an incredible photograph -- showing the bullet hurtling through the air above Trump’s shoulder, dangerously close to him ... and from his POV, it's likely the bullet that nicked him in the ear. However, that's not quite confirmed yet.

Doug told "CBS Mornings" he thought the defining shot of the whole thing was Trump defiantly raising his fist as he was escorted offstage. But when he looked back at his photos, he realized he had a golden nugget -- a pic of the bullet that probably left Trump bloodied.

He added he’s been covering Presidential events since '83 and never imagined something like this could happen.

But it looks like his Sony digital camera, which can capture up to 30 frames per second, delivered quite the historic snapshot, despite the shocking circumstances.

As we've reported, Thomas Matthew Crooks was taken down by snipers Saturday after wounding ex-Prez Trump ... killing one person in the crowd and injuring 2 others.

TMZ obtained video of Crooks as he began shooting ... 130 yards from Trump. He was shot and killed within a minute of the time he began firing.