The first patent granted for a "training machine" was patent #1064968 in 1913. This is the first that resembles a modern treadmill for people and designed as a machine to enhance your fitness. There is no evidence to say that this application was commercially viable. When you see the images within the patent you can see why. It looks very complicated and has many moving parts.
The modern day treadmills were arguably invented by Dr. Robert Bruce and Wayne Quinton at the University of Washington in 1952. They were designed to help diagnose lung and heart problems. Treadmills for medical uses can be found everywhere for their help in rehabilitation as well as helping diagnose coronary artery disease.
In 1968 Dr. Kenneth Cooper wrote a book called "aerobics" that sold 30 million copies around the world. Further than that, over the years, he has had over 20 books published many of which promote the cardiovascular and other health benefits to aerobic exercise. Apart from running outside a treadmill is the perfect way to achieve these benefits in the comfort of your own home.
Today the treadmill has become the biggest selling fitness equipment by a long way. They are quite big and the average US home is perfect in size for these devices. Unfortunately you have to assemble these yourself or you can pay extra for treadmill assembly. This can cost anything up to $300. I always advise you pay for treadmill assembly for many reasons.
Treadmills are often the most used equipment in gyms. I have experienced a queue when I have worked out and they now have a maximum of 10 minutes per use. You can now buy manual treadmills which have no motor. These are often less expensive and often need little to no assembly. Treadmills are not going to go away anytime soon as people realize the benefits you can get from aerobic exercise.