Product design projects involve multiple team members with various knowledge and competencies that have to evolve with time, due to rapid organizational, technological and marketing changes. Project managers require management methods to optimize the assignment of tasks to human resources according to their competency levels and the need for competency evolution. In this paper, we tackle this optimization problem in the context of multiple design projects and multiple periods. The model we propose seeks to minimize two function objectives: (i) minimize the supplementary salary cost due to the extended task duration when using under-competent employees and (ii) minimize the financial penalties when the competency goals have not been reached. This problem has been solved by means of a simulated annealing -based optimization algorithm.