The demand for flexibility in the configuration of highly customized capital goods such as aircrafts is rising. Customers request specifying product options later than required by the currently defined order fulfilment process of the OEM. However, late changes of previously configured products can cause disturbances in global production networks.In this paper, a modular-based approach is presented, allowing customers to specify options just-in-time depending on the respective lead times following an Engineer/Order-to-order (EOTO) strategy. The concept of Just-In-Time Specification with its respective phases of order specification and steps of production planning is described and applied to the aircraft manufacturing industry.