In the email seen by news agency Reuters, Musk told employees that it was time to bring the Tesla Semi to "volume production".
Tesla shares soared to more than US$1000 following the news, making it the second most valuable car company in the world, sitting behind Japanese giant Toyota. This is despite Tesla having yet to turn an annual profit.
Hydrogen-electric truck competitor Nikola also saw a significant gain in their stock price after the company went public, with shares soaring by 93 per cent and making the newcomer more valuable than Ford.
In his Tweet Milton took a thinly-veiled swing at Tesla's Cybertruck, writing: "You'll get to see a real operating truck, not a fake show truck. Expect stamped metal panels, functioning interior w/ hvac, 4x4, etc".
Nikola's stock jumped a further 20 per cent after the Tweet, before reversing most of those gains.
In April Nikola and IVECO announced a joint venture, offering a hydrogen-electric prime mover called the Nikola Tre, that's expected to be released to North American and European markets next year. No announcement has been made for an Australian release.
Nikola has two additional models of prime mover trucks in the works which are expected to compete directly with the Tesla Semi.
Both companies are named after the Serbian-American electrical engineer Nikola Tesla, who invented the alternating current (AC) electric motor – the type used to power electric vehicles.