OTTAWA - Lockheed Martin will be cutting it close if it intends to deliver F-35 stealth fighters to Canada on schedule in 2017, according to a revised timetable released by the U.S. defence giant.

An extended flight test and software programming plan was ordered by the Pentagon over a year ago and the changes mean the aircraft will not exit its full development phase until late 2016.

The aircraft-maker, the world's largest defence contractor, is scrambling to hire over 100 software engineers to complete the three-stage development of computer programs that will fly and control the advanced stealth fighter in combat.

A senior company official said the version of the F-35 Lightning II that Canada wants to buy -- the A model -- should have its final set of software codes by early 2016. Another variant to be purchased by the U.S. Marine Corps is now slated to get its final software by October of that year.

Industry observers say that's an optimistic assessment given that engineers will have to write up to 4 million additional lines of code to complete the 8 million lines that go into a fully functional F-35.

Even still, the official insisted the deadlines will be met.

"International deliveries will not be affected," Steve O'Bryan, Lockheed Martin's vice-president of business development for the F-35, said from Washington.

After the United States, Australia and Italy are the first two customers in line for the fighter in 2014. But O'Bryan said the aircraft will be delivered with training software, which will be upgraded.

Britain, Turkey and Israel are slated to get the next round in 2015. Canada has only signalled its intention to buy the F-35 and has yet to sign an agreement, but the Conservative government said it intends to begin taking delivery in 2017.

In addition to software, the new stealth fighter, which has become a political lightning rod in Canada for its estimated $14-billion to $16-billion price tag, will have to undergo an additional 2,000 test flights, bringing to 7,800 the number needed for the aircraft to considered fully developed.

The Liberals have threatened to scrap the purchase should they win the next election.

One of the big selling points the air force has trumpeted is the sophistication of the aircraft and the automated systems that aid the pilot, such as an uninterrupted data stream that allows maintenance technicians to monitor the aircraft from the ground.

The development improvements were ordered by U.S. Defence Secretary Robert Gates as frustration mounted over delays in the estimated $382-billion defence program. Lockheed Martin was ordered to reallocate roughly $4.6 billion from production to development in order to rigorously test the aircraft.

Alan Williams, a former assistant deputy minister of materiel at National Defence, says the issue of software development is significant because most weapons systems today depend on the quality of their computer programs.

With the F-35 software yet to be fully written and tested, "we don't know what the aircraft will be capable of doing," he said.

A spokesman for Defence Minister Peter MacKay said Tuesday that the government remains confident the fighter will be ready for service when air forces takes delivery and the timeline is flexible.

"The development of Canada's variant of the F-35 remains on track for delivery between 2016 and 2022," Jay Paxton said. "This government will buy this aircraft at the peak of production to ensure we get the best plane for the Canadian Forces at the best price for Canadian taxpayers."

Williams, who has been a strident critic of the purchase, said the schedule change reinforces his argument that Ottawa should wait until the aircraft is fully developed before signing on the dotted line.

The issue of delays is secondary because there are always delays in big ticket military purchases, he added.

"The fact that there are delays shouldn't be surprising," said Williams. "Any cautious person would be cautious of these dates. Based on history, based on experience, much more can go wrong."

"Why commit to something that's in a developmental state? You don't when it will be ready. You don't know what it's capabilities will be at the end, notwithstanding the promises. You do not know what it's going to cost."

O'Bryan said Lockheed Martin accepts the criticism that the original timetable for developing the aircraft systems was too optimistic and the schedule now is "much more realistic."

The program has built in time for troubleshooting potential glitches.

"We're confident. So far in flight tests, the software has been stable, much more stable than legacy programs," O'Bryan added.