The blaze, thought to have been caused by a mechanical fault, broke out in the rear wheel arch of the 85 service as it approached the roundabout on the south side of Corbridge bridge.
After evacuating passengers from the bus, which travels between Carlisle and Newcastle, the driver raised the alarm at 12.40pm.
Passers-by reported hearing a loud bang as the heat caused one of the rear tyres to explode.
Fire crews from Hexham and Tyne and Wear were on the scene within minutes and fought the flames before the vehicle was recovered.
Police closed the roundabout and bridge to traffic and pedestrians for an hour and a half.
Area managing director for Arriva North-East Nick Knox said: "We are working with Northumberland fire and rescue service to investigate exactly what happened.
"Safety is always our top priority and we are conducting a thorough internal investigation into the cause of the incident at Corbridge."
Watch video footage of the incident from Corbridge resident, Bill Ferguson, below.
<iframe width="560" height="315" src="https://www.youtube.com/embed/jfwrVyoHu1E" frameborder="0" allowfullscreen></iframe>
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