In 1921, Tulsa Oklahoma, planes were used to drop bombs on the city of Tulsa.
Now I am no genius, but I am quite sure planes cannot be bought in retail stores, and especially ones with historical significance. So I assume the US govt knew. If so, despite this being a very different time, why was this allowed?
There is a lot of information on the Web about the Tulsa Race Riots of 1921, much of it very clearly meant to incite rather than to inform. However, that aside, one common point seems to be that several crop-dusting aircraft were impressed into service and used to drop improvised dynamite bombs on the black portion of Tulsa. These were most likely military surplus training aircraft flown from the Curtis-Southwest Field outside of Tulsa. Some sources note (without documentation) that these planes could have been flown by National Guard troops who had been mobilized to put down the riots, but most are silent as to who actually flew the planes. In any case, it does not seem that the assault used military planes and military bombs, as implied by the original poster.
Lots of references available: Greenwood Cultural Center: http://www.greenwoodculturalcenter.com/
The Tulsa Race Riot of 1921 by I. Marc Carlson: http://tulsaraceriot.wordpress.com/
A Report by the Oklahoma Commission to Study the Tulsa Race Riot of 1921: http://www.okhistory.org/research/forms/freport.pdf
Tulsa Race Riot Archive at OSU-Tulsa: http://libguides.osu-tulsa.okstate.edu/content.php?pid=472496&sid=3867515
Tulsa Race Riot Archive at the University of Tulsa: http://www.utulsa.edu/mcfarlin/speccoll/collections/RaceRiot/
Airplane manufacturers don't have retail stores because they use different sales system, but anyone with the cash can order one. In the early years, they sold through distributors. Curtiss-Southwest Airplane Company in Tulsa was one such distributor for Curtiss aircraft. They had a service center, and offered air freight and advertising services. In Mississippi Valley Magazine, Volume 2, March 1920, the president of Curtiss-Southwest talks about his company and sales to the oil industry in Texas and Oklahoma.
Furthermore, in 1921, you were just as likely to find surplus WWI aircraft for sale. The US Government sold thousands of surplus Curtiss JN-4's to the public for about $1,500. Many of these ended up in the hands of former WWI pilots and started the barnstorming craze.
So, there would have certainly been privately-owned aircraft in Tulsa at the time of the Riots.