His abduction, 12 months ago, outside a hotel gym in the country's main city, Dar es Salaam, sparked a huge manhunt.
Speaking to the media for the first time about his ordeal, Mr Dewji told the BBC that he was blindfolded and disorientated when he made the plea.
He was released after 10 days and says no ransom was paid.
"I was blindfolded and there were times they kept on threatening me with guns to my head and five days, six days into it I was thinking I was losing my eye sight," Mr Dewji told BBC Focus on Africa's Audrey Brown.
"[The kidnapper] was like 'I'm going to shoot you' and I was like 'you can shoot me and kill me', because I was losing it. You get disoriented, you're tired, because it's a form of torture."
He said he could hear people looking for him.
"I kept hearing these chopper noises and thinking that maybe this chopper's looking for me."
Ten days after his abduction in October 2018, his company, MeTL, tweeted a quote from Mr Dewji saying that he had "returned home safely".
He has since thanked people for praying for his safety.
'No ransom paid'
Mr Dewji told the BBC that his kidnappers had abandoned him in a field 15 minutes drive away from where he had been abducted at the Coliseum Hotel.
The authorities have
not established a clear motive for the abduction and Mr Dewji is also in the dark about it.
"I still don't know why it happened," he said. "Obviously the intention looked like it was money that they wanted. In the end they left me without the money."
He believes the kidnappers gave up due to pressure built up from media and political attention.
A Tanzanian taxi driver, Mousa Twaleb, has been arrested and is awaiting trial. But he is the only suspect that is being held.
'Need to give back'
Mr Dewji believes there were around three or four foreigners from Mozambique and South Africa involved, who he believes have fled the country.
The billionaire says he has made some changes in his life; replacing the lights in his house with brighter bulbs and has stopped jogging alone on the beach.
He said being kidnapped has made him reflect that "you need to give back".
Mr Dewji is credited with turning his family business from a wholesale and retail enterprise into a pan-African conglomerate. MeTL has interests in textile manufacturing, flour milling, beverages and edible oils in at least six African countries.
Forbes magazine estimated his net worth at around $1.5bn (£980m), which would make him Africa's youngest billionaire.
He intends to invest almost $400m in agriculture in Tanzania in the next 24 months, not for profit but to make an "impact", he said.