The crash happened at around 12:45 p.m. at the Vectren Air Show at Dayton International Airport. No spectators were hurt.
A video posted on WHIO-TV shows the plane turn upside-down as the performer sits on top of the wing. The plane then tilts and crashes to the ground, exploding into flames as spectators screamed.
Ian Hoyt, 20, an aviation photographer and licensed pilot from Findlay, was at the show with his girlfriend. He told The Associated Press he was taking photos as the plane passed by and had just raised his camera to take another shot.
"Then I realized they were too low and too slow. And before I knew it, they hit the ground," he said.
He couldn't tell exactly what happened, but it appeared that the plane stalled and didn't have enough air speed, he said.
"I'm still shaking," Hoyt said. He said he had been excited to see the show because he'd never seen the scheduled performer - wing walker Jane Wicker - in action.
The names of those killed weren't immediately released, but a schedule on the event's website and the announcer at the show said Wicker was performing.
Airport spokeswoman Linda Hughes and Ohio State Highway Patrol Lt. Anne Ralston confirmed the deaths to the AP.
The show was canceled for the rest of the day, but organizers said events would resume Sunday. The National Transportation Safety Board said it is investigating the crash.
Another spectator, Shawn Warwick of New Knoxville, told the Dayton Daily News that he was watching the flight through binoculars.
"I noticed it was upside-down really close to the ground. She was sitting on the bottom of the plane," he said. "I saw it just go right into the ground and explode."
Wicker's website says she responded to a classified ad from the Flying Circus Airshow in Bealeton, Va., in 1990, for a wing-walking position, thinking it would be fun. Her full-time job was as a budget analyst for the Federal Aviation Administration, according to her website.
She told WDTN-TV in an interview this week that her signature move was hanging underneath the plane's wing by her feet and sitting on the bottom of the airplane while it's upside-down.
"I'm never nervous or scared because I know if I do everything as I usually do, everything's going to be just fine," she told the station.
Wicker wrote on her website that she had never had any close calls.
"What you see us do out there is after an enormous amount of practice and fine tuning, not to mention the airplane goes through microscopic care. It is a managed risk and that is what keeps us alive," she wrote.
In 2007, veteran stunt pilot Jim LeRoy was killed at the Dayton show when his biplane slammed into the runway while performing loop-to-loops and caught fire.
Organizers were presenting a trimmed-down show and expected smaller crowds at Dayton after the Air Force Thunderbirds and other military participants pulled out this year because of federal budget cuts.
The air show, one of the country's oldest, usually draws around 70,000 people and has a $3.2 million impact on the local economy. Without military aircraft and support, the show expected attendance to be off 30 percent or more.