Service Oriented Architecture (SOA) is a common design pattern that allows building applications composed of several services. It promotes features as interoperability, scalability, and software reuse. Services composing a SOA system may evolve and change during runtime, often outside the control of the owner of the application, which makes the verification and validation processes complex. Among all the automated techniques to validate the behavior of an SOA application, is Model-Based Testing (MBT). MBT requires an accurate model of the application in order to generate suitable test cases. However, the intrinsic of a SOA application sets significant challenges to MBT effectiveness. In this paper we discuss the challenges in the testing of SOA applications, and we propose the use of Model-Driven Engineering (MDE) to improve the flexibility of testing tools. Finally, we outline our plan for realizing MDE-driven MBT within an existing online testing framework.