Digital twins, especially when standardized, are an essential aspect of Industry 4.0, because they enable interoperability for components of different companies, both in the engineering and operational phase. The German initiative "Plattform Industrie 4.0" considers the Asset Administration Shell (AAS) as the Digital Twin for Industry 4.0. The concept provides a meta model and initial submodels, each of which contains the information needed for a common use case. For a variety of use cases, in particular customer specific applications, the information that may need to be provided by multiple AASs to ensure the correct execution of the application must be specified by the partners involved. This contribution presents an approach and an architecture for a model-based generation of AASs. The foundation is a model containing the specification of a use case. Each partner involved in the execution of this use case uses an instance of the presented Digital Twin Generator to create the required AAS. The Digital Twin Generator analyzes the required information based on the provided model, finds it in a company's tools, in databases or on the physical twin and publishes the generated AAS on a web server.