Based on the M1919 machine gun, the M2 was the big brother. The cartridge and the gun were designed simultaneously. It's another Browning design, although the finished product required work that was completed after JMB's death. The result was a machine that has been in continuous service since it's inception and is in use by the U.S. military today. My son rode behind one in the turret of an MRAP in Afghanistan. Those young men in the picture above were behind the same gun 70 years ago.
The history, use, and variants can, and have, filled volumes. But the basics look like this.
It weighs about 85 pounds, it's putting out 450-600 rounds per minute, and it's accurate out to about 2,000 yards. They can be had, there are some transferable models around. I didn't find any currently for sale, but $40,000 might get you one. Then you have to feed it. .50 BMG in belts. About $3.50 a round, and did I mention 450+ rounds a minute?
Just to touch on the other part of rinfanti's wish list, there are about 45 flyable B-17s left. The cost of the plane and the guns might be doable with that 50 million dollars, but getting the permission to mount a pair of these guns in firing condition to go flying would be about as likely as finding the Holy Grail.