Assurance cases are used to argue in a structured, and evidence-supported way, that a property such as safety or security is satisfied by a system. In some domains however, instead of single systems, product lines with many system-variants are engineered, to satisfy the needs of different customers. In such context, single-system methods for assurance-case creation suffer from scalability issues because the underlying assumption is that the evidence and arguments can be created per system variant. This paper presents a novel method for product-line assurance-case creation where all the arguments and the evidence are created without analyzing each system variant. Consequently, the effort to create an assurance case scales with the complexity of system variants, instead with their number. The method is based on a contract-based design framework for cyber–physical systems, which is extended to define the conditions under which all system variants satisfy a particular property. These conditions are used to define an assurance-case pattern, which can be instantiated for arbitrary product lines. Moreover, the defined pattern is modular to enable step-wise assurance-case creation. Finally, an exploratory case study is performed on a real product-line from the heavy-vehicle manufacturer SCANIA to evaluate the applicability of the presented method.