May I refer people once again to this podcast of D. P. Davies recounting some of the early problems when certificating the DH Comet (47 mins in) and the Boeing 707 (1hr 14mins in). Sometimes, designers and manufacturers take short cuts and sometimes the regulatory authority becomes 'too close' to the manufacturer. The 737 has morphed a long the way from a small shorthaul aircraft to something much larger, more powerful and with considerably greater range. This old 1960s design has needed many add-ons and modifications to achieve this. I do wonder whether this was a wise path to follow, and I also wonder whether the FAA has neglected to maintain sufficient distance when approving some of these changes.
https://www.aerosociety.com/news/aud...nnia-brabazon/