The paper describes the design of a distributed software architecture for an intelligent planning system based on a process model of cognitive systems (PMCS). PMCS is described in terms of its problem-solving behaviour and structure, and is illustrated through a simple case study in robotics. In the PMCS model, autonomous knowledge sources, each implementing a component of the system's functionality, co-operate by message-passing in order to maintain a distributed model of the system's environment and to synthesise decisions based on the model. Simulation tools developed at the University of Westminster for the rapid prototyping of parallel systems have been used to simulate the software architecture based on PMCS. Simulation results for the planning case study are presented in order to demonstrate the effectiveness of PMCS in solving planning problems in a distributed manner.