Owner Troy Whalen said Mr. B was "acting goofy" one day while Whalen was working from home, prompting the owner to investigate, the Chicago Tribune reported Thursday.
"I thought maybe someone was breaking into the house, or maybe a huge bird flew through the window. I don't know, you think those sorts of things, you know?" Whalen said.
By the time Whalen reached the front of the house, flames were shooting in through a window -- and Mr. B "freaked out and ran back upstairs," Whalen said.
As he called emergency services, Whalen followed Mr. B upstairs, telling the 911 operator he had to rescue his rescuer hiding in the closet.
Grayslake Deputy Fire Chief Greg Formica credited Mr. B, who lost his rear left leg as a pup, with Whalen's easy escape.
"My new personal philosophy is, if a three-legged dog barks, listen," he said, telling the newspaper that Mr. B has gotten a whole lotta petting and treats since news of his rescue bark broke.