An evaluation of a hybrid-electric commercial transport aircraft was conducted by analyzing the simulated performance and lifecycle CO2 emissions of a conventional single-aisle commercial transport aircraft and an aircraft with a modified parallel hybrid-electric propulsion system. A flight performance simulation was developed in a MATLAB/Simulink environment using publicly-available aircraft data for the Boeing 737-700 commercial transport aircraft. A parallel hybrid drivetrain, modeled after the CFM56-7B26 turbofan engine, was integrated into the aircraft performance model. Various missions with different degrees of hybridization and battery energy densities were simulated and compared to the conventional turbofan case. CO2 emissions associated with fuel burn and electricity generation for charging battery systems were modeled for each hybrid aircraft configuration. The results indicate that reductions in CO2 emissions per passenger mile are achievable using a parallel hybrid propulsion system, though many hybrid-electric configurations were observed to actually increase emissions, relative to the conventional configuration. A candidate near-term propulsion system configuration was defined, which utilizes a 50% hybrid architecture and a battery energy density of 700 Wh/kg. This configuration was estimated to produce 19.0% less lifecycle CO2 emissions than a modern baseline aircraft, with a max range accessible to over 85% of all global flights, making it a viable option for environmentally-responsible aviation.