Case-Based planning can fruitfully exploit knowledge gained by solving a large number of problems, storing the corresponding solutions in a plan library and reusing them for solving similar planning problems in the future. Case-based planning is extremely effective when similar reuse candidates can be efficiently chosen. In this paper, we study an innovative technique based on planning problem features for efficiently retrieving solved planning problems (and relative plans) from large plan libraries. Since existing planning features are not always able to effectively distinguish between problems within the same planning domain, we introduce anew class of features. Our experimental analysis shows that the proposed features-based retrieval approach can significantly improve the performance of a state-of-the-art case-based planning system.