Here's what I found on Wikipedia about the history of Part 25:
Part 25
This part contains airworthiness standards for airplanes in the transport category.
Transport category airplanes are either:
Jets with 10 or more seats or a maximum takeoff weight (MTOW) greater than 12,500 pounds (5,670 kg); or
Propeller-driven airplanes with greater than 19 seats or a MTOW greater than 19,000 pounds (8,618 kg).
A rather important section of this part, is the 121 - climbing guaranteed with one engine out for multi-engine aircraft.
The Boeing 737 and later types, and Airbus A300 series, are well-known airplane types that were certified according to standards set out in FAR Part 25.
Most of the Federal Aviation Regulations, including Part 25, commenced on February 1, 1965. Prior to that date, airworthiness standards for airplanes in the transport category were promulgated in Part 4b of the US Civil Air Regulations which was in effect by November 1945. Effective August 27, 1957, Special Civil Air Regulation (SR) 422 was the basis for certification of the first turbine-powered transport airplanes, such as the Boeing 707, the Lockheed Electra, and the Fairchild 27. SR 422A became effective July 2, 1958, and was superseded by SR 422B, effective August 29, 1959. Only a few airplanes were certified under SR 422A, such as the Gulfstream I and the CL-44. First generation turbine-powered transport category airplanes such as the DC-8, DC-9, and B-727, were originally certified under SR 422B. SR 422B was recodified with minor changes to 14 CFR part 25, which became effective February 1965.