As others have stated, the term "bought the farm" was common at least early in the '40s (by my recollection) and it appeared to me to have been known and used by those two generations previous to me.

While it was used quite a lot by military fliers in WWII and later, I always had the impression that an earlier usage was connected to the "dirt poor" farmers who had no hope of "owning the farm," due to the debt incurred to keep running one, and the "6 foot by three" farm in which they finally rested was at last theirs, and couldn't be claimed by the mortgage holders.

While it was common enought for the government to pay some compensation to a farmer whose crops were damaged by an errant plane - regardless of survival by the pilot, the payment (in cases in which farmers discussed it in "town meeting" in front of the pool hall on Saturday night where I listened) seldom was for more than the crop for that year, and a one-year-in-a-row "successful" crop payment didn't pay off the mortgage for many farmers.

I can definitely affirm that the usage predates the existence of any "jets" in the US military.

People who use a euphemism of this sort often don't much concern them selves with an "original meaning" if they have heard it, and can find a meaning in some context that makes sense to them in the time and place where they use it. An "earliest usage" for a term by one group of people and with one meaning doesn't necessarily disprove an earlier usage by others with a different meaning.

The older description of a military person or unit having "seen the elephant" on a mission was used, although not widely, in the WWII era by persons who most likely had no knowledge of what it meant to early settlers well before Civil War times.

John