This study is about contract car manufacturing (CCM) projects in a case company. The aim is to find out if it is possible to create a flow model that will fit to these projects in general and that could support the flowing execution of the project. The target is expressed in the hypothesis of this study: It is possible to create a common flow model for a CCM project and that can be applied to different project cases in the same context. The research is based on three literature domains, Design Science, Project Management, and Systems Thinking. They all are present in complex engineering projects execution but have seldom been considered together in previous research. The analysis of the four case projects showed that a general flow model for CCM projects is plausible and can be implemented in all projects. Furthermore, this modelling principle can be adapted to other kinds of projects as well. The deliverables only need to be formed accordingly.