Currently, the electric sedan gets an EPA-rated 87 miles of range, using 28 kWhs of its 36-kWh battery, though its now-standard "Range Plus" package takes advantage of more of the pack, stretching that driving to 100 miles or so. It may take a year or several – the model just had a refresh – but expect those distance digits to double.
The rumored range is said to be 500 km (310.7 miles), but since that would likely be calculated using the European drive cycle, expect something significantly lower stateside; say 220-ish official miles. This would be in keeping with expectations from a number of battery-powered competitors in the 2017-2018 time frame. It is unknown if this supposed next-gen B Class Electric Drive is in any way related to the rumored "Ecoluxe" program.
The Tesla/Daimler relationship stretches back to 2009, when the Stuttgart-based outfit made a significant investment in the California automaker, quite possibly saving it from extinction in the process. We have reached out Mercedes-Benz about this report, but as of this writing, have not had a reply. We will update this post when we hear back.