The Boeing Co. announced plans yesterday to develop a "stretch" version of its new 767 twin-engine jumbo jet that will carry 40 more passengers and fly 1,000 miles farther than current versions.

There are no firm orders for the new plane, to be called the 767-300, but Boeing marketing executive Myron Anton said in a news conference here that three airlines have expressed interest. One of them, Delta, has announced it is working with Boeing on development of the plane.

Boeing estimates the market for the new jet at 250 to 300 planes at a minimum over the next eight to 10 years. The 767-300 would sell for about $48 million, or $8 million more than the current 767.

Anton downplayed the importance of Federal Aviation Administrator J. Lynn Helms' speech last Friday in which Helms said he is not prepared to certify two-engine jetliners such as the 767 for transoceanic flights. Three jet engines are required for such passenger-carrying flights under current regulations so that at least two engines would be available if one engine failed.

Anton presented charts and graphs to show that the new 767-300 and a planned extended-range version of the basic 767 could fly from New York to many European cities and meet present FAA rules. To do so would require a longer, more northerly route than is normally used.

Nonetheless, Anton said of Helms' comment, "I think we face a lot of challenges in the world, and this presents another challenge."

Maximum takeoff weight of the new plane will be 345,000 pounds, up from the current 300,000 pounds. The fuselage will be stretched a total of 220 inches--110 on each side of the wing--and structural improvements will be made to landing gear, wing and fuselage. Fuel tanks will be expanded.

The result will be a plane that carries 254 passengers in a typical first-class, cabin-class configuration and as many as 290 passengers for a high capacity charter flight.

Boeing claims that, on a per-seat basis, the 767-300 will be 20 percent more fuel efficient than the Airbus Industrie A300-600, the competitive aircraft from the European consortium. "You can see the impact of later technology," Anton said.

Deliveries on the 767-300 could begin in the spring of 1986, he said.