Boeing 737-300 US AirBoeing 737 300 US Air airplane model. The 737 300 is the first of the three member second generation CFM56 powered 737 family, which also comprises the stretched 737 400 and shortened 737 500. The success of the second generation Boeing 737 family pushed sales of the mark to over 3000, a record for a commercial jetliner. Boeing announced it was developing the 737 300 in March 1981. This new variant started off as a simple stretch over the 737 200 but