Jet aircraft
Jet aircraft are aircraft with jet engines. Generally, civil jets fly at altitudes of 30,000 feet to 45,000 feet or so, since they are most economical to operate at these altitudes. Jets are not economically efficient at low altitudes, which is why propeller powered airplanes are commonly used for very short routes.
The first jetplane was the Coanda-1910 (Romania), piloted by its inventor Henri Coanda in 1910. Unfortunately, the aircraft crashed during its first and only demonstration.
The second jetplane was the Heinkel He 178 (Germany), piloted by Erich Warsitz in 1939.
The first jet fighter was the Messerschmitt Me 262, piloted by Fritz Wendel. It was the fastest conventional airplane of WW II (the rocket-powered Messerschmitt Me 163 Komet was faster). Mass production started in 1944, too late for a decisive impact. About the same time Britain produced the Gloster Meteor which made up the first operational jet fighter squadron in 1944 and which saw limited action in WWII.
During the Korean War on November 8, 1950, United States Air Force Lt. Russell J. Brown flying in an F-80, intercepted two North Korean MiG-15s near the Yalu River and shot them down in the first jet-to-jet dogfight in history.
BOAC operated the first commercial jet service, from London to Johannesburg, in 1952 using De Havilland Comets.
Modern civil jets generally cruise at speeds of .75 to .85 Mach, which is to say, 75 percent to 85 percent of the speed of sound. The speed of sound is a function of air temperature, and therefore the speed of a jet is not really "constant" in terms of miles per hour, but varies by atmospheric conditions. In general, modern civil jets cruise at about 420 to 580 miles per hour or so.