Anthony Sanders, 28, from Essex had been travelling along the dual carriageway on Boxing Day after going shopping in the sales.
The object — which Mr Sanders thought was the same shape and size as a house brick — was thrown as he approached the footbridge just past the North Camp slip road towards Aldershot.
In an attempt to avoid the missile, Mr Sanders swerved into the central reservation and wrote off his car, a six-year-old Saab convertible.
“My first thought was that’s it — I’m dead,” he said.
“I was so worried that the car would flip over, and I wasn’t too sure how strong the roof would be as it’s a convertible.”
Mr Sanders smashed into the barrier at around 60mph, causing it extensive damage, although he miraculously escaped with only bruises.
“I was extremely lucky,” he admitted. “I’m very glad that no-one was coming up behind me at the time.
“I had been in the process of overtaking a car on the inside lane when I saw two people on the bridge above and they threw something towards me.
“I thought I was going to hit the other car if I moved to my left. So, in trying to miss it, I completely oversteered and hit the central reservation.”
The front of Mr Sanders’ car caved in when he hit the barrier, and the driver he had been overtaking stopped to try and help him.
“I think the other driver was as shocked as I was to see that I was actually all right,” added Mr Sanders.
“The car definitely didn’t look good.”
A motorist on the opposite carriageway also witnessed the crash and spotted the two youths on the footbridge above the road.
He immediately stopped his car and sped up to the bridge to catch the boys.
Two youths were later arrested by Farnborough police.
The pair, who cannot be named for legal reasons, have both been interviewed and are on police bail.
Investigating officer Pc Mark Chapman said: “This was extremely dangerous and frightening for Mr Sanders.
“It is very similar to an incident on the M3 around five years ago that nearly killed a driver, when youths threw a concrete block off a bridge on to his car.”
Pc Chapman would like to speak to anyone who was travelling along the A331 in either direction between Aldershot and North Camp on the afternoon of Boxing Day, Thursday December 26, and may have seen people on the bridge.
He can be contacted at Farnborough police station on 0845 0454545.