Quality of Service (QoS) management has become an important requirement for middleware platforms and distributed applications. Early QoS engineering attempts focused on single-category extensions to standard middleware. In recent years generic middleware frameworks have been proposed in order to facilitate multi-category QoS management. Our experience with such a framework has revealed that QoS management cannot be shielded from the applications. Applications need to be designed in a QoS-aware manner and application QoS requirements need to be translated into appropriate middleware extensions. Therefore we propose a comprehensive, model-driven development process for QoS-enabled distributed applications on top of QoS-enhanced middleware platforms. Our approach builds on OMG's Model-Driven Architecture (MDA). We demonstrate the mapping from the platform-independent to the platform-specific model, and we show how the design approach is applied targeting a .NET-based QoS-enabled middleware.