When was military tanks invented?

"The first tanks were designed by Leonardo da Vince in the late 15th century. However, the first Military Tank was developed in early 20th century after the development of Armed trains."

I don't understand why you have replaced a comprehensive answer with a Wiki-lite, inaccurate one. Da Vinci designed a wheeled, muscle-powered, wooden vehicle with a protective covering for the crew. It was not a tank. It was a covered fighting vehicle, like those that had been in use for centuries. The sooner the world rids itself of the idea that da Vinci was responsible for the tank, the better.

The tank became a possibility only after the development of armour plate, caterpillar tracks, and the internal combustion engine, in the late 19th and early 20th centuries.

Several plans for vehicles very similar to tanks (some of which would be called self-propelled guns nowadays) were drawn up between 1904 and 1912; by the French Captain Levavasseur, the Austrian Lieutenant Gunther Burstyn, the Australian engineer Lancelot de Mole, and the Russian naval engineer Vasiliy Mendeleev, amongst others. Burstyn's idea was the only one that incorporated a revolving turret. However, none of them was actually built.

After a few months of WWI, both Britain and France, separately, began to look for ways of overcoming the deadlock of trench warfare, and various machines were produced. Eventually it was decided in both countries that the best solution was an armoured vehicle moving on caterpillar tracks. In Britain most of the early development was conducted by the Landships Committee, a body set up by Winston Churchill and officially part of the Royal Navy. It was later taken over by the Army, and a series of heavy and medium tanks was built.

In France, similar things took place at roughly the same time. Two types of medium tank were developed in 1916 (400 of each), and the first tank with a turret, the light Renault FT, appeared in 1917.

The British were the first to use tanks in action, but France produced and used more than everybody else put together, about 3,000. In 1918 Germany produced 20 of her own, and used some captured British ones.

You might feel inclined to change this back to your own version. If so, you will be perpetuating ignorance, and it is not worth my spending my time on it.