Yes, Air Force One (the call sign for any aircraft carrying the President of the United States) is heavily armored and considered a "flying fortress." The aircraft (currently modified Boeing 747-200Bs) features bulletproof and blast-resistant windows and is shielded with armored plating to protect against ground-level small arms fire and even man-portable missiles. Beyond just being "bulletproof," the plane is designed to survive the electromagnetic pulse (EMP) of a nuclear explosion. It is equipped with advanced electronic countermeasures to jam enemy radar and "flares" to divert heat-seeking missiles. While the specific armor thickness is a state secret, the aircraft is essentially a secure, mobile command center designed to keep the Commander-in-Chief safe from nearly any conventional or unconventional physical threat while in the air.