KC-10 versus C-10
The C-10 designation was initially given to the (then) Handley-Page Jetstream. But, when H-P was going through some rough financial times prior to being absorbed into BAe, the USAF cancelled the contract before any deliveries were made to operational units. The number was retired and then assigned to the next bird coming along, the Douglas DC-10 ATCA (Advanced Tanker Cargo Aircraft). I hadn't thought of it before, but you are correct in saying that no US aircraft has had the K prefix without having first had a primary designator (eg: B-/KB-29, A-/KA-3, C-/KC-135), I recall no controversy over that since the aircraft was bought from the get-go to serve both roles.
An interesting sidelight about the designation, however: I was the USAF Program Manager on the Air Staff from 1979-83. When Strategic Air Command (SAC) got the money to buy the new ATCA, the folks in Military Airlift Command (MAC) were a bit miffed because they felt that it was "their" money that bought the planes. This is all a bit esoteric, but the KC-10 was bought with Force Program (FP) 2 (tactical, MAC/TAC) money rather the FP 1 (strategic, SAC) funds to keep it distanced from the B-1. (Many on the Hill were accusing the USAF of buying a new bomber that really was much more costly than advertised because, they claimed, it needed additional tanking capability as well. Buying tankers with FP 1 money would have played directly into this argument.)
MAC never would quite relent on the aircraft. They went so far as to work out deals with SAC to ensure that the KC-10, when needed to augment the strategic airlift force (C-5/-141), would be available. But, more than that, I even ended up going head-to-head with CinC MAC over getting about 20 of the birds transferred directly to MAC and renaming them CK-10s! Oh, the joy of a young major taking on a four-star! In short, I won. SAC kept all 60 as KC-10s. When needed, the KC-10 would be operationally chopped to MAC control (such as when flying a specific MAC cargo mission on what was known as a MAC Channel flight). To this day, it worked and is working just fine. The bottom line is that we didn't have to change 16 trillion pages of TOs to read "KC/CK-10," which is what would have happened if that insanity had ever gone through!
The Tech Order numbering system for the KC-10for example, electrical powerwas TO 1C-10(K)A-2-24. That meant that as far as the TO people were concerned, the basic designation of the aircraft type was C-10, even though I recall nothing else official ever using that designation. The data block and all documentation has always read KC-10A. ( Scott A Willey, Col USAF (Ret), 12/15/99)