Existing design and development methods do not meet designers' and developers' needs. They are difficult to learn and to use; they are complex, sequential and rigid and thus far from being adapted, reliable and efficient. This paper presents M2Flex, a process metamodel for highly supporting flexibility. M2Flex is based on a recent definition of flexibility along four dimensions: (1) versatility, the ability of the metamodel to provide various equivalent choices, (2) granularability, the possibility of defining components with several levels of details, (3) completeness, the possibility of defining optional components and pre-defined reusable results and (4) distensibility, the capacity of the resulting process model to be extended or reduced at runtime. This paper shows how M2Flex is original by the flexibility it offers to designers and developers at runtime.