The paper presents the design and modeling of a twenty-channel, bidirectional WDM-PON architecture with IM/DD for the 5G fronthaul in C-RAN systems. The proposed architecture is fully compliant with the ITU-T G-series Supplement 66 requirements on next-generation fronthaul interfaces. Each channel has a data-carrying bit rate of 25 Gbps, thus yielding a total capacity of 500 Gbps for the entire system. The entire design has been simulated in OptiSystem V.22. The analysis of the BER for different lengths of fiber and continuous wave laser power conditions provides the performance evaluation. Through simulation results, it can be confirmed that the system conforms to the F1 and Fx functional split specifications that are under consideration for split options 1 through 7a, using 25 Gbps in each direction. The results also indicate that the system can work efficiently over 11.5 km of bidirectional single-mode fiber without any need for DSP or optical amplifiers. The simplicity, low cost, and conformance to the standard fronthaul requirements make the proposed WDM-PON configuration a very strong candidate for the deployment of short-reach 5G fronthaul applications, featuring high-capacity, bidirectional connectivity, and reliable performance in C-RAN operational environments.