Build-to-order series production is gaining increasing importance as markets demand customer-specific product variants. Orders have to be assigned to plants and periods in global production networks and then to lines and cycles. Consequently, respective workloads as well as supplied materials have to be balanced due to limitations in resource capacities. As first step planning defines the solution space for second step planning, this paper introduces a mathematical model for order assignments to plants and periods anticipating assignments to lines and cycles. Given that orders are not fully specified for first step planning, the approach includes provisions for dealing with uncertainty.