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Delay-Tolerant/Disruption-Tolerant Networks (DTNs) have been proposed to cope with the challenges of communication in some extreme or special environments. Due to the uncertainty of node mobility, application traffic demand and other factors, it is difficult to provide performance guarantee for a DTN where all nodes may move arbitrarily. With controlled mobility, message ferries can be utilized to guarantee the network performance. Central to the problem is the design of ferry routes under some constraints. In this paper, we consider the delay requirements in DTNs and study the multiple ferry route design problem with delay constraints. We formulate the problem as a global optimization problem and give the conditions when a solution is valid and when there is no valid solution to the problem. Then a heuristic algorithm is proposed to find the minimal number of ferries and the ferry routes subject to the delay bounds. The algorithm performance is evaluated and compared to the SIRA algorithm. The experiment results have shown that the performance of a DTN can be guaranteed using a reasonable number of message ferries moving along the ferry routes found by the proposed algorithm.