Are aerial tankers converted aircraft or their own designs?

Question

For instance, this KC-46. Is this a converted Boeing 767? Or its own design?

Same with this Rivet Joint.

Answer 1

The Airbus A330 MRTT (Multi Role Tanker Transport) is actually built on the same assembly line as the regular passenger A330s in Toulouse and then flown to Spain for modification:

The A330 MRTTs are based on Airbus’ popular A330 widebody passenger airliner, with the aircraft produced on the company’s commercial airplane final assembly line in Toulouse, France. Once their initial built-up is complete, they are flown to Airbus’ military aircraft facility in Getafe, Spain to be transformed with hardware and systems for their dual roles as an air-to-air refuelling platform and an airlifter for troops and cargo. [...]

During the conversion, Airbus teams install some 16,000 types of new components and approximately 450 new electrical harnesses (for a total cabling length of more than 50 km.), as well as 6,000 brackets and 1,700 connectors.

^{(airbus.com - Airbus transforms A330s into Multi Role Tanker Transports through an optimised conversion process)}

The cabin can be configured in different ways, similar to how a passenger A330 could be configured based on the customer's need. The cargo hold is also available with the same ULD containers as the passenger variant:

Serving three missions with one capable platform, the A330 MRTT can carry up to 111 tonnes of fuel in the aerial refuelling mission – the highest capacity of any tanker aircraft. [...]

Requiring no reconfiguration, the A330 MRTT also serves as an efficient transport aircraft, carrying passengers on the main deck and cargo in the lower lobe. Seating capacity is up to 300 passengers on the main deck, which also can be arranged in a medical evacuation configuration that accommodates 40 stretchers and 20 seats for medical staff, along with 100 passengers. The lower lobe – which accommodates up to 37 tonnes of cargo, can be loaded with 27 LD3 containers or eight military pallets.

^{(airbus.com - Multirole Tanker - Defence History)}

Such a mixed configuration might look something like this:

^{(airbus.com - A330 MRTT)}

Answer 2

The tankers are "based on", not a regular passenger jet "converted to".

The same engines and basic body, but the whole internals are different.

The Wiki page for the KC-46 outlines this https://en.wikipedia.org/wiki/Boeing_KC-46_Pegasus

Some, but not all, of the other platforms are ex-airliners. https://www.air-and-space.com/Boeing%20707%20Military.htm

"EC-18B, 81-0894 at Vandenberg Air Force Base on June 9, 1990. It carries Boeing construction number 19583 and was delivered to American Airlines as 707-323C, N8403 on November 28, 1967. The Air Force acquired it on July 15, 1982. "

The E-3B AWACS platforms were purpose built by Boeing.

Answer 3

I agree with WPNSGuy that most tankers are derivatives of transports, sometimes also bombers. But there are exceptions for sure.

The Dash 80 prototype led to the commercial 707 and the military KC-135 tanker. Both planes shared the basic design of the Dash 80 but were very different airplanes, neither one being a derivative of the other. One great difference was in the width and length of the fuselage. Airlines wanted the 707 fuselage to be 4 inches (2.5 centimeters) wider than the tanker’s.....

Source: https://www.boeing.com/history/products/707.page

So KC-135 and 707 are parallel but separate designs. In fact, military version of 707 is designated as C-137 or C-18 instead of C-135 and Boeing's internal designation of KC-135 would be Model 717

Answer 4

One counter-example : KC-390 (Embraer) doesn't seem based on an existing airframe, but designed concurrently as tanker and/or military transport from the ground up.