Over the course of history humanity rode several kinds of animals to war: horses, camels and elephants being the most common. However, bears are not amongst them.

So I was wondering, if they could be properly domesticated and/or trained, would bears be feasible as ridable animals in wartime situations?

A bear is a lot more fierce than a horse or dangerous than a camel, but less big of a target than an elephant. This of course also has to take things into consideration like being able to charge, marching speed, their diet, endurance and other such things.