Lufthansa was the launch customer for Boeing’s original 737-100 jetliner.
The German airline placed an order for the new short- to medium-haul twinjet in 1965. Lufthansa introduced the 737 into scheduled passenger service on April 10, 1968, helping establish the aircraft as a practical European workhorse.
The 737-100 was the shortest production version of the first 737 generation. Boeing later developed the slightly longer 737-200, which became more widely used. The 737’s compact size, two-person flight deck, and rear-mounted engines made it well suited to routes that did not require a wide-body aircraft.
This answer is sometimes confused with Pan Am, an important Boeing customer that helped launch other aircraft programs, including the 747. United Airlines also became a major 737 operator, but it was not the launch customer for the original 737-100.