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