Want to know one of the most common Keurig-related questions we're asked?
What's the difference between Keurig's B series and K series brewers?
Answer: Nothing. The switch from B to K was solely a marketing decision as the company realized branding their brewers with a K made more sense. (They're called Keurig, after all.)
Discontinued and replaced by the K15, the B31 truly is mini. Its water reservoir needs to be filled before each use up to its 10 oz max brew size.
The B31 measures less than 11" in every direction with a somewhat narrow 6.9" width that was great when it was first released but seems bulky with the release of Keurig's new 4.5" K-Mini.
Keurig has released over 70 different coffee makers over the years, and only about a dozen of them are still in production and supported by Keurig. Those in-production models include both the K15 and K-Mini, making both preferable to the discontinued B31.