Essential Products Inc., the smartphone maker founded by the creator of Google’s Android mobile software, confirmed it has a new $300 million war chest as it prepares for the seemingly insurmountable task of taking on Apple Inc. and Samsung Electronics Inc.

The startup on Wednesday unveiled the large roster of investors taking a chance on it, including Chinese internet company Tencent Holdings Ltd. and Amazon Inc.’s Alexa Fund. Essential also disclosed that Best Buy Co. stores and Amazon.com Inc. will be its retail launch...