We propose a process model for the development of formal and semi-formal specifications based on the notions of multi-view states and development operators. A specification state is composed of a UML and a B view. The development of a specification is seen as a sequence of application of operators, which model design decisions and make both views evolve. To produce consistent specifications, we define a consistency relation between views, allowing to define and check operators' correctness. Thus, the development process guarantees that the specification can be safely verified.