This paper describes an interactive object-oriented system for damage tolerance design of stiffened panels. Its ultimate goal is to free the engineer from most of the tedious and error prone tasks, so that he can concentrate on the actual design problem and find optimum solutions by experimenting with the design variables in a straightforward, intuitive manner.The panel configuration is represented in an object oriented fashion, using a high level of abstraction and in terms of objects well known to the designer such as: stiffeners, cracks and boundaries. Input data is performed interactively, through a user friendly windows-based interface. Crack propagation analysis is performed with the Boundary Element Method (BEM). The BEM meshes are automatically designed using a knowledge based strategy.