I've just finished reading an at times disturbing book on the airborn assault on Sicily. It is clear that the USAAF C-47s used in that operation were not equipped with armour or self sealing tanks. Was the C-47/Dakota ever so equipped?
I am not aware of any transport aircraft of the era that where designed with crew protection. You have to remember that they are not combat aircraft. Yes they can be used for glider towing or paratroop transport. But the design goals of a transport aircraft are to carry the biggest payload for the longest distance possible.
Adding armor means that any additional weight added comes out of the useful load, either cargo or fuel. Self sealing tanks also add weight, but they also decrease the available fuel that can be carried. So now if you have added say 1000lbs to each aircraft, decreased either the load by that much, or decreased the range of each aircraft, or a combination of both payload & range. How many more aircraft are required to perform the same mission?
I would say that it is like why modern military transport planes do not have ejection seats.
In my notes, I have mention of "An initial order was placed for 1000x SSFT kits for the C-47 in May 1944".
Again in my notes, I have mention of "a significant number" of US C-47s having been fitted with SSFT beginning in February 1945, just prior to Operation Varsity. Most of the 437th TCG C-47s assigned to the Operation were fitted with SSFT and had the wings vented. A significant number (but not all) of the 436th TCG had also been modified before the Operation.
I have not been able to find out the total number of airframes fitted with SSFT, either prior to Operation Varsity or afterward.
In "Green Light!: A Troop Carrier Squadron's War From Normandy to the Rhine" by Martin Wolfe, he mentions that his squadron - the 81st TCS (Troop Carrier Squadron) - managed to get the last 12 out of 16 aircraft fitted with SSFT just before Operation Varsity.
The British were also supplied with a number of SSFT for their Dakotas, but I have not been able to find out how many or when.
Just found out that the book I mentioned above, "Green Light!: A Troop Carrier Squadron's War From Normandy to the Rhine" by Martin Wolfe, is now available from DTIC and a couple of other history websites. It is available as a free pdf download and is word-searchable:
I briefly went through it again and found mention of some C-47s with SSFT being deployed in September 1944 for Operation Market Garden. There was a limited number of SSFT available at the time and Wolfe mentions that in the 436th TCG only the group and squadron leaders aircraft had been fitted with them.