The "Big Bertha" was a giant novelty slot machine introduced by Bally Manufacturing Co. in 1931. 837 giant machines were released under the name.
The additional reels and their size meant more reel symbols. This allowed for a greater degree of randomness, meaning operators could offer higher jackpots (the three-reels of the era were often limited to 15 reel symbols, limiting maximum payoffs to the hundreds, since payouts were pretty frequent).
Big Bertha was highly successful when it debuted and was soon replaced by Super Big Bertha, an even grander deluxe slot machine with eight reels and 20 symbols per reel. This machine cost $150,000 to manufacture, which greatly limited its availability, plus a five-horsepower motor to power the reels.
The problem with the Big Bertha machines (besides the cost of buying one) was the odds of hitting a jackpot, estimated at one in over 15 billion spins.
The Super Big Berthas -- and other copycat large-reeled machines -- were made obsolete by the random number generator, which meant that slot machines with high jackpots did not have to be huge and expensive - you could control the payouts via a chip. Still, a few casinos retain them for the novelty value (a couple of years ago or so, Bally's/Paris actually rigged theirs to enable a customer to use it to propose to his girlfriend, via a message on the reels. She accepted.)
Here are the ones we still know about in Las Vegas:
