Billionaire Elon Musk's The Boring Company has sold all 20,000 of its flamethrowers in just five days.

The entrepreneur announced Sunday that his tunneling firm had started selling flamethrowers. The devices were available for pre-order at a price of $500 each, and customers were also able to buy an "overpriced" fire extinguisher with a "cool sticker" for $30.

By Monday, the company had sold $3.5 million worth of flamethrowers. Musk's idea to start selling them followed The Boring Company's unusually successful run selling hats. He said last month that, if the company sold 50,000 hats, it would start selling flamethrowers.

The Boring Company's new product officially sold out Thursday, Musk said on Twitter. That means Musk's infrastructure firm has now sold $10 million worth of the devices.

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The flamethrower starts shipping in the spring, according to The Boring Company's website. Musk added on Twitter that the flamethrowers would ship with a "complimentary Boring fire extinguisher."

The billionaire, who is also the CEO of car manufacturer Tesla and aerospace company SpaceX, posted a video to his Instagram account Sunday where he was seen wielding one of the devices and charging toward the person filming him.