Abstract: This paper presents the concept and prototype implementation of a drone for Multi-purpose Autonomous eXploration of indoor environments – MAX. The purpose of MAX is to support first responders in the difficult task of assessing unknown and potentially dangerous or hostile situations in indoor or underground environments. The approach for addressing challenges associated with this task has been to construct a custom-designed drone based on requirements and conditions of first responder missions. This paper reports on the first phase of development of the MAX drone, aimed for experimentation with autonomy functionality in first responder contexts and for enabling further development of advanced higher-level planning functions. It describes the overall design of the MAX drone, its capabilities in terms of robust positioning and autonomous mission execution, along with the status of key enabling algorithms for exploration, such as target point selection and path planning.