Situated multi-agents systems (MAS), and other agent- based systems, are often complex. Formal reasoning is needed to ensure their correctness and structure their development. Event-B is a formal method with tool support allowing a stepwise development of reactive distributed systems. We propose using Event-B to helpful the specification and the safe development of situated MAS. In this article, we mainly report our experience with the Event-B stepwise development of a situated MAS which study the movement of vehicles in a convoy. This article also aims at serving as a guide for the development of other MAS, taking agents- specific features into account.