Originally Posted by
Airbanda
Boeing 707-320 series. The engines are Pratt & Whitney JT4A turbojets. The version was quickly superceded by the 320B which had first generation turbofans - P&W JT3D. Some 320 series aircraft were re-engined with the later type but others found there way to second-hand market and carriers such as Donaldson.
As I understand it, none of the 707-320 aircraft were converted to fan engines, as that was not technically possible, I seem to recall airframe as well as engine considerations. The slightly smaller 707-120, and the smaller again 720 did have conversions made available by Boeing, which a number of operators took - American Airlines had a large fleet of both these early models, and converted the lot. Those of the Monarch fleet at Luton that came from American included these.