Requirements engineering is nowadays the broadly accepted method to manage a customer's requirements. The result is a specification from which a solution is implemented and which is used to validate the realization in terms of their fulfillment. However, today's tools assist in organizing and tracking the requirements but reliable criteria about their completeness, consistency, and realizability are missing. Furthermore, the resulting artifact is a document, which must be read and understood by humans, which itself is error-prone. It is obvious that errors and ambiguities result in an unwanted solution which is often and in the worst case only discovered in the final stage: Testing. This paper outlines an approach for constructive requirements modeling, which describes completely a customer's demands in a formal manner so that already during the requirements' elicitation inconsistencies are eliminated, completeness is assessed, realizability is ensured, and all valid test cases can be derived by using a model-based testing approach. Therefore, we propose adaptions to the traditional V-model to not only save valuable development and testing time but also to achieve better results. The applicability is shown on the example of the software for an auxiliary heating system at a large German OEM.