These models were used in the mid 1960s development of the Boeing 737. Boeing had been studying short-haul jet aircraft designs and wanted to produce another aircraft to supplement the 727 on short and thin routes. They were not alone in this pursuit as rival aircraft such as BAC 1-11, Douglas DC-9, and Fokker F-28 were already further along down the road. Preliminary design work began on 11 May 1964 and Boeing's market research yielded plans for a 50- to 60-passenger airliner for routes 50 to 1,000 mi long. This was initially even smaller than the DC-9 Series 10 which would enter service just the next year in 1965. Lufthansa became the launch customer for the 73 on 19 February 1965 with an order for 22 aircraft after the airline, it was reported, received assurances from Boeing that the 737 project would not be cancelled. This was a real concern as the 737 was a slow seller initially. As development continued with Lufthansa, raised the seating capacity of the 737 to 100 seats. In 1965, United joined into the program with a stretch version called the Boeing 737-200s. This became the much more popular variant.