If you believe the "official history", the first recoilless gun, which is known to be actually built, was created in 1910. Allegedly, it was then that the commander of the US Navy Cleland Davis invented a weapon that works on a recoilless principle (with the release of counter-mass - like the SPG-9), and is designed to arm aircraft.
In 1911, he received a US patent for this invention No. 1108714. This weapon was used on an experimental British bomber from the Handley Page O / 100 series to combat enemy submarines.
Cleland Davis continued to improve the weapon called the Davis recoilless cannon and patented their modifications with detailed descriptions until 1921."
However, the real story, and not the one that is imposed to be considered "official" - is quite different. If you wish, you can find completely different historical data concerning when the recoilless gun was first invented - in 1844 and moreover, it was with a telescopic sight!
If in 1910 the weapon invented by Davis was used exclusively for installation on airplanes, then in 1844, recoilless weapons were fired from hands
I found information on this recoilless gun in The Illustrated London News, published October 26, 1844. This historic document convincingly proves that recoilless cannon technology has existed since at least 1844.
“The gun that attracted the most attention was a recoilless cannon from which a strong charge of gunpowder was fired several times without any recoil (no more than 6 mm).
The inventor claims that this principle applies to weapons of any caliber. A telescope is attached to the gun with which the gunner can accurately hit even the smallest target.
During the demonstration of the cannon, several shots were fired at the "huts" and each time they were completely destroyed. This experiment caused a long (about an hour) discussion among the scientists and professional gentlemen who were present at the demonstration, until the inventor Duke of Normandy took out the gun from the gun carriage and fired, holding it in his hands, as after him another gentleman and the shots took place without any recoil, and the targets were amazed.
Then the inventor said to one of the staunchest skeptics, "I will let you take my gun apart and if you reveal a secret, I will give it to you, but if you don’t open it, then pay me." The skeptic was not ready for such a bet and refused.