Slotted-waveguide arrays (SWA) have been employed in radar, remote sensing, and communication applications where narrow-beam or shaped-beam patterns are required. One of the most common configurations for resonant SWA places longitudinal slots located in the broad face of a rectangular guide which is fed by one end and is shorted at the other. However, the relative bandwidth achieved in these arrays is about 1%. This paper present the design, optimization and simulation of a two-layer centrally-fed waveguide slot array antenna with ten slots (2×10) operating at 12 GHz, that achieves a bandwidth of 4% (440 MHz) at a return loss level of-14 dB. The design was a two-step process, first meeting the bandwidth requirement, and then a sidelobe level (SLL) of 20 dB. To achieve the SLL, the excitation coefficients were improved with a least-square optimization approach. The resulting radiation diagram computed from the models run in Matlab were verified with CST Microwave Studio, which takes as input the physical dimensions of the array and the locations of the slots. The design requirements were satisfactorily met.