We present a novel specification-based approach for generating tests for products in a software product line. Given properties of features as first-order logic formulas, our approach uses SAT-based analysis to automatically generate test inputs for each product in a product line. To ensure soundness of generation, we introduce an automatic technique for mapping a formula that specifies a feature into a transformation that defines incremental refinement of test suites. Our experimental results using different data structure product lines show that incremental approach can provide an order of magnitude speed-up over conventional techniques.