United Airlines Boeing 737-900ER N17403 taxiing at Chicago O'Hare International Airport with distinctive split scimitar winglets in the Evo Blue livery.

United Airlines Boeing 737-800 (N26226) prepares for departure at Denver International Airport under clear blue skies.

Taxiing in the early morning or golden hour, this Southwest Airlines Boeing 737-800 wears the vibrant 'Heart' livery with split scimitar winglets in front of a Frontier Airlines hangar.

United Airlines Boeing 737-800 (N34217) taxiing at the airport, viewed past distinctive split scimitar winglets.

United Airlines Boeing 737-800 (N27213) taxiing at San Francisco International, sporting the globe livery with blended winglets.

A United Airlines Boeing 737-900ER taxiing in golden hour light. The aircraft sports the modern Evo Blue livery and distinctive 737NG blended winglets.

United Airlines Boeing 737-800 (N77538) at the gate, ready for boarding at a major U.S. airport.

Air Canada Boeing 737 MAX 8 at the gate, featuring the airline's striking new black tail livery.

United Airlines Boeing 737-800 (N27426) taxiing with split scimitar winglets at a major U.S. airport.

A United Airlines Boeing 737-900ER in the 2019 Evo Blue livery is seen taxiing at San Francisco International Airport, with a Dreamliner and other United jets in the background.

United Airlines Boeing 737-824 (N27275) taxiing at a major US airport in the modern Evo-Blue livery.

United Airlines Boeing 737-800 (N37464) taxiing at an airport, showcasing the latest Evolution Globe livery.

United Airlines Boeing 737-800 (N37566) prepares for departure in bright morning light.

A United Airlines Boeing 737-800 (N17233) rests at the gate under the night sky.

United Airlines Boeing 737-800 taxiing at golden hour, sporting the latest globe livery.

United Airlines Boeing 737-800 taxiing at golden hour, with distinctive blended winglets and a city skyline in the background.

American Airlines Boeing 737-800 (N973NN) taxiing under dramatic skies.

A rare American Airlines Boeing 737-800 in special AirCal (heritage) livery taxiing at a major U.S. airport.

Alaska Airlines Boeing 737-900ER 'Honoring Those Who Serve' special livery, registration N265AK, taxiing at the gate.

An Alaska Airlines Boeing 737-800 is seen taxiing at golden hour, with ramp activity in the foreground.