Firefighters rushed to the scene and pumped retardant over the flaming vehicle as it sat in the shoulder off the westbound lanes.
About a dozen passengers who had been aboard the bus were able to get off safely, and none of them were injured.
The bus, operated by Merrill Gardens Northgate retirement home, was completely destroyed. As the flames and smoke were extinguished, there was nothing left but the bus' hollow, burned-out hulk.
Merrill Gardens spokeswoman Lori Wright said the bus was transporting a private wedding party to the east side.
More information will be published as it becomes available.
The following video of the fire was posted by Alex Trzyna on YouTube. He said the heat from the fire was so intesne, he could feel it through the walls of his car, two lanes over.
Another video was posted by Ian Linkletter on YouNews at komonews.com: