Final answer:
Modern cockpit doors in commercial airliners are designed to be bulletproof as part of enhanced airline security measures following September 11, 2001.
Step-by-step explanation:
After the events of September 11, 2001, airline security measures were significantly enhanced, including the reinforcement of cockpit doors. Modern cockpit doors in commercial airliners are indeed designed to be bulletproof and to withstand other forms of attack. This reinforced design is standardized across airlines as part of the global aviation security enhancements. The modification of cockpit doors to make them bulletproof helps to prevent unauthorized access and enhances the safety of flights globally.