Buy (Bought) it-
From early military aviation. In Burchfield, Oxford English Dictionary Supplement, and Lighter, Historical Dictionary of American Slang.
1920- (ref. to WW1): The wings and fuselage, with fifty-three bullet holes, caused us to realise on our return how near we had been to "buying it."
1945, Huie in "Omaha to Okinawa"- A bomb dump went up...and...Palmer was one of those who bought it."

Buy (bought) the farm-
To crash fatally, especially U. S. Air Force.
1954, Jet glossary in NY Times: Bought a plot (had a fatal crash)
1955, in American Speech vol. 30: Added to U. S. Air Force Dictionary in 1956.
"Jet pilots say that when a jet crashes on a farm the farmer usually sues the government for damages done to his farm by the crash, and the amount demanded is always more than enough to pay off the mortgage and then buy the farm outright. Since this type of crash (in a jet) is nearly always fatal to the pilot, the pilot pays for the farm with his life."
1963 quote: "Look at the hole he made. He bought the back forty like a plumb bob."

Military, WW2: "One of those in the guts and we've bought the farm, I'll tell you!"

Now common in American and Canadian speech.