This is about the best story of the evolution I've come across
Because only a relatively small number of the 707s would be built with the Conway, provision was made for a replacement engine to be flown out on a scheduled passenger flight to an airplane grounded with engine problems at a distant airport, enabling the latter airplane to return to its home base in London. This was achieved through the use of a special streamlined pod and pylori that could be quickly attached to the wing inboard of the number 2 (port inner) engine. Boeing’s Brien Wygle and FAA (Federal Aviation Agency) pilot Sliff flew the pod certification flight with N31241 (G-APFB) on November 10, 1959; G—APFH was flown with a pod on its delivery flight to Prestwick, Scotland, in july 1960. On February 12, 1960, the Model 707-400 Series was awarded FAA certification; however, a protracted and contentious delay arose from the differing certification requirements of the FAA and the British Air Registration Board (ARB). The ARB's concerns were the potential for stall during takeoff through an excessive angle of rotation—which had caused two Comet 1 crashes—and the 707’s Dutch roll characteristics, that had led to three training accidents in 1959, with fatal results in the case of American Airlines and Braniff international Airways (Airways, October 2009).
Accordingly, the ARB required a demonstration of the aircraft's ability to unstick at a nose—high attitude, at various speeds. Nowadays a normal part of the flight testing for any new type, VMU (velocity maximum unstick) takeoffs were conducted at Edwards Air Force Base, California, during November 1959, using BOAC’s first 707. These tests showed that the 707 would only unstick after the nose was lowered. A modification resulting from these tests saw a 39in (lm)-deep ventral tail fin added to alleviate the possibility of a premature rotation—and also improve the airplane’s longitudinal stability.
The low-speed stability fix took much longer, and included an addition of 35in (89cm) to the top of the vertical stabilizer and duplication of the yaw damper, during the takeoff/climb and approach/landing phases. To counter additional engine thrust, a modification of the rudder control system—allowing the rudder to be fully powered throughout its whole range of movement (not only the first 15 degrees) - had already been incorporated on the Dash 300 and Dash 400 models.
Thus G—APFB was modified at the Boeing plant, and after a final series of test flights by the ARB’s chief test pilot David P Davies (of Handling the Big Jets fame)—who had personally insisted on the changes—British certification was awarded on April 28, 1960.
Ivor Lusty,the airline’s plant representative at Renton, ‘accepted' G-APFD in an 'informal ceremony' on April27, and following transfer of title (the official delivery) the next day, the 707 was flown nonstop to London, arriving on April 29. Captain T B (Tom) Stoney, BOAC's 707 flight manager, was in command for the 4,900mi (7,885km), 9hr 44min flight.
The taller vertical stabilizer became a standard feature of the entire 707/720/KC-135 series. All 707-300/-400 series also had the ventral fin, as well as Dash 100s with Pratt & Whitney JT3C turbojets and 15 low gross weight Dash 300Bs (with the ]T3D turbofan), which had a 17 flaps takeoff setting. Because they had a full set of leading edge flaps, 14 flaps for takeoff, a '’series’ yaw damper, improved stall warning (stick shaker) operation, and aerodynamic upgrades to the wing, the Dash 300B ’Advanced’ and ’Advanced-Heavy', and -300C did not require the ventral fin. A smaller (13in/33cm) ventral fin was fitted to Dash 100Bs (with turbofans) and 720s to prevent damage in over-rotation, as the stick shaker did not activate early enough.
Boeing supplied retrofit kits without charge to operators that had already had taken delivery of 707s, an action that helped cement the manufacturers reputation in airline circles.
https://rentonwa.gov/uploadedFiles/L...20combined.pdf