DTRON: a tool for distributed model-based testing of time critical applications. Cyber-Physical Systems (CPS) present the greatest challenges but also the greatest opportunities in several critical industrial segments such as electronics, automotive and industrial automation. Governing the complexity and design correctness issues of CPS software requires methodologies and tools that address the problems of intrinsic concurrency and timing constraints over a large spectrum of CPS architectures. In this paper we present DTRON, a framework for model-based testing that addresses the issues of distributed execution and real-time constraints imposed by the design of networked CPS. DTRON extends the Uppaal model checking tool and online test execution tool TRON enabling coordination, synchronization, and online distributed testing. The notion of $Delta$-testability required to guarantee the controllability of distributed tests is one of the main design considerations for DTRON. The core part of the paper presents the architectural solutions for implementing DTRON and then special focus is put on the performance evaluation of the tool taking into account the communication and test adapter delays in networked systems. We demonstrate that the co-use of Spread message serialization service and Network Time Protocol allows reducing $Delta$ down to the 1 ms range, which is sufficient for testing timing properties of a substantial class of networked CPS. We exemplify the applicability of DTRON with three distributed testing case studies, namely, city street light controller network, interbank trading system, and robot navigation system.