It is a 737-9 Max variant.
MAXimum comfort and efficiency: Our first 737 MAX 9 takes flight
By The Hub team , June 08, 2018
This week, we became the first North American carrier to operate the fuel efficient Boeing 737 MAX 9 aircraft. The brand new plane entered service as United flight 686 between our hub at Houston George Bush Intercontinental Airport and Orlando International Airport in Florida. On Thursday, the MAX operated flights between Houston and Anchorage, Alaska and Houston and Austin, Texas.