Here's the story of the early Douglas Aircraft commercial and military twins -- the DC-3 and its predecessors:
-- Douglas started with the DC-1 in 1933 (not pictured). They only built one but it was the basis for the follow-on DC-2 of 1934, which was a commercial success and was also bought in small numbers by the military. The Army Air Corps had a number of designations for the DC-2: C-32, C-33, C-34, XC-38, C-39, C-41 and C-42. (Whew!) The Navy and Marine Corps also bought a few as the R2D-1. A total of about 200 DC-2 variants were sold -- pretty successful for the Depression 1930s. The photos include a DC-2 airliner, an Air Corps C-33 and an Air Corps C-39 military version of the DC-2 1/2.
https://en.wikipedia.org/wiki/Douglas_DC-2
Having achieved success with the DC-2, Douglas then further developed the aircraft as the Douglas Sleeper Transport (DST), considered to be the first of the DC-3 line. The concept was to fly passengers overnight with sleeping accommodations in the form of bunk beds, or to accommodate seated passengers in airline seats. The DST featured small windows for those in the upper bunks in addition to the regular row of windows along the side of the airplane.
The DC-3 really opened the floodgates of orders. In addition to domestic U.S. airlines, a large number of foreign airlines ordered aircraft. The military also ordered variants with a double-wide cargo door on the left rear, a reinforced floor for cargo and other detail changes, designating the result as the C-47 Skytrain. Once again, the Navy and Marines got in on the action with orders for the R4D-1.
After Pearl Harbor, most U.S. airline aircraft were taken over by the Army Air Force. Just to further confuse the designations, these were C-48s, C-49s, C-50s, C-51s, C-52s, C-68s and C-84s (!) Power for the aircraft came from either Wright R-1820 9-cylinder radials or Pratt & Whitney R-1830 14-cylinder radials, accounting for some of the variation.
Further improved C-47 models were introduced; for details, see the Wikipedia article. A version of the C-47 designed strictly for passenger transport -- and lacking the cargo door, etc. -- was produced in smaller numbers as the C-53 Skytrooper. Navy and Marine versions continued with the R4D designation.
https://en.wikipedia.org/wiki/Douglas_DC-3
https://en.wikipedia.org/wiki/Douglas_C-47_Skytrain
The Douglas design was also manufactured under license in the Commonwealth countries as the Dakota, in Japan as the Imperial Japanese Navy L2D and in the Soviet Union as the Lisunov Li-2. A large number of nations operated civil, military and both U.S.- and foreign-manufactured aircraft, but the U.S. story is sufficiently complex that I will not address these. Suffice it to say that a huge number of aircraft were built: Over 16,000.
The C-47 Skytrain was the primary aircraft used worldwide during World War II for dropping airborne forces by parachute and for towing troop-carrying gliders used in airborne assaults. A large number were also used for high-priority personnel and cargo; the ex-airline DC-3s performed this function in the continental U.S.
After World War II, the impressed civil DC-3s were returned to the airlines and large numbers of C-47s were declared surplus or transferred to allies. In the initial postwar period, the DC-3 continued to dominate shorter routes, although its four-engine big brother, the DC-4 (military C-54) (not pictured) handled the longer-range routes.
The late 1940s and early 1950s saw newer improved aircraft introduced to the market and Douglas saw an opportunity to upgrade the DC-3/C-47 as those aircraft were in plentiful supply. The DC-3S (S for Super) was the result. Douglas was unable to find takers commercially or in the Air Force, but the Navy ended up buying 100 remanufactured aircraft as the R4D-8. These had revised, squared-off tail surfaces, a fuselage stretch, slightly swept wings and other improvements.
Another interesting development of the Navy versions was use in Antarctica. Both earlier R4D-5L and the later R4D-8L versions were adapted for skis and used on the ice.
The Vietnam war of the 1960s-70s brought further modifications. USAF C-47s were used for SIGINT collection (EC-47N, P and Q) and gained fame as the first gunship -- the AC-47D Spooky armed with three side-firing 7.62mm miniguns.
By the mid/late 1970s time had finally caught up to the DC-3 and its military cousins. Gas turbine-powered aircraft dominated the market and the DC-3 was increasingly found only in the third world or in collector's hands. Over the years, a few were even converted to use turboprops.
My own experience with the C-47 was slightly exciting -- in 1965 my parents and family went on vacation to Japan and Taiwan. Our return trip to home in the Philippines was via the base's C-47H and as we neared home, a fire caution light came on in the cockpit. We were met upon landing with several firefighting trucks. I think the light was a faulty indicator and we deplaned more or less normally.
