We propose a process model for the development of specifications based on the notions of multi-view state and of operators. A specification state consists of a UML view and a B expression of the same specification. Operators model design decisions as the simultaneous transformation of UML and B descriptions. A specification development is a sequence of applications of operators. We define a notion of coherence of a specification state which allows us to define formally a notion of correctness for operators. Thus, the development process guarantees that the specification can be safely verified and validated.