The Boeing 737 has been the best-selling jet airliner in aviation history, with about 1,000 currently flying in the air at any time. Succeeding the 727, the 737 was announced as the "twin-feeder airliner to complete the family of Boeing passenger jets." Although over 60% of the structure and design of the 727 was transferred over, most obviously its wide fuselage, the biggest design question to differentiate it from its already popular contemporaries at the time, like the Caravelle and DC-9, was where to put the engines. The decision was made in 1966 by Joe Sutter himself to mount them on the wings. This change had many advantages: it reduced interference drag, improved flying position, increased cabin space, lowered the engines for better access for repairs and maintenance, and required less materials, such as wiring. The fin had to be enlarged, however, to take the weight of the engines, and small structural adjustments had to be made to smooth out any aerodynamic interferences. Since then, Boeing 737 has branched out various series: the 737 Classic, Next Generation, Max, and the P-8 Poseidon, its version for the Navy.
The 737 was also the first 2-crew aircraft by Boeing, which helped conclude the debate between airlines, pilots and engineers, and their unions. After several months under FAA supervision, it was declared at the end of 1967 that a 2-person crew was safe to fly a commercial aircraft.
One problem that has still persisted to this day, however, has been its complicated airstairs that for decades have frustrated workers so much that they have even hastily removed them on a few occasions to reduce delays.
The Boeing 737 has had an impressive run sales-wise--breaking a record 10,000 sales as a commercial jet in 2012.