Critical systems (e.g. safety-critical and security-critical systems) need the highest levels of assurance. The effective engineering design of critical systems still lacks easy-to-use, practical and above-all trustworthy tools which allow the exploration of possible design strategies, and support formal reasoning about their critical properties. In this paper, we describe the Veracity prototype tool, aimed at providing support for modelling and reasoning about state machine designs for critical software-based devices. The tool has three main components: a graph editor, for constructing state transition diagrams; an animator, for exploring symbolic execution of the machine; and a prover, for verifying critical properties of the machine.