You are probably all aware of the infamous Bob Semple tank, from New Zealand. They were described in the Evening Post as "Powerful machines". When first introduced to the public in March 1941 this is what the newspaper had to say:
"[...]and as tanks they have immense power. Not only can they climb a grade of 1 in 2, but they will travel through water over four feet deep, traverse an embankment four and a half feet high, smash through gorse hedges, scrub, and saplings up to six inches in diameter, and move across country where roads do not exist. Their armament consists of a number of quick-firing guns. Each tank has a crew of eight, and normally carries 25,000 rounds of ammunition. In addition there is room for the carriage of troops and ammunition in safety over country that might be under enemy fire."
This attempt at public relations soon failed as the public caught their first glimpse of the Bob Semple. It was an ungainly machine lashed together with whatever materials could be sourced for its armour. The rapid firing guns were nothing more than machine guns.
However mad the idea, it did have some merit. The Bob Semple bodies were stored around the country and could be fitted to any local tractors in short order if the Japanese invaded, and in most cases a moving protected machine gun is never a bad thing to have, as long as one understands its limitations.

The news story however highlights another interesting thing, the Bob Semple wasn't the first of its kind. The article in the news starts out:

"The genesis of these 25-ton tanks was a photograph taken in the United States and given to Mr. Semple."

Now one can't be sure,but I think we can make an educated guess as to what that photograph was of. I believe it may have been a Disston Tractor Tank. However there is, as always, conflicting information on the subject. Originally designed during the Great Depression, lots of companies were considering how to sell to the world when everyone was in a bad way financially. Sources differ, some say it was the Caterpillar company, others that it was Disston themselves that came up with the idea, however the result was a Caterpillar model 35 tractor with with an armoured body made by Disston, a company well known for quality steel products, with an emphasis on safes.
What may come as a shock is that there were at least two versions of the Disston tank. The first early version had turret. Later models were simplified, used a shorter length track and had the turret replaced by a small gun shield that protected the gunner.
Armament in both cases seems to have been a 37mm M1916 infantry gun (very similar to the gun of the FT17). Along with that the Disston had at least one .30 Calibre machine gun.
What may surprise you even more is that the Disston tank actually found some buyers. From 1923 until 1927 the only tanks the USMC had were single digit numbers of M1917 6 ton tanks. After those were removed in 1927 it wasn't until about 1933 when the USMC brought a number of Disston tanks. Some sources say six, others sixteen.

By 1935 the worst ravages of the Great Depression were receding, and the Disston company had to alter their marketing. They now pitched their tank as something that could be assembled in under two hours, implying that it didn't need to remain as a front line vehicle. Again this seems to have been aimed at less affluent countries. One order was placed by China, although the order seems to have been cancelled in 1935. In the same year the first deliveries were made to Afghanistan. The exact number delivered isn't known, but at least five can be identified in photographs. Some sources suggest completed tanks and a smaller number tank bodies were delivered.
Four Disston's in Afghanistan
Despite Afghanistan's turbulent history at least two Disston's still survive and as far as I can tell are still awaiting rescue in Kabul scrap yards.
There is one other group of tractor tanks left to talk about from the period. However sources for them are even harder to find than stuff on the Disston. In Russia during Operation Barbarossa a factory at Odessa is reported to have converted several tractors into tanks, which had mixed effects on the German invaders. The sources are so sparse, and have so little detail its almost impossible for me to verify it. So what I think I'll do is link to the Wikipedia page on the subject and you can make your own mind up.
What is certain is there are a large number of photographs of Soviet tractor tanks, of different models. So its likely that some did see combat to some degree.