Mitsubishi Aircraft Corp., which made World War-era fighter jet Zero, said Thursday that delivery for the first Japanese-made passenger jet has been delayed by a year, making it the fourth time the Mitsubishi Regional Jet has been delayed. ANA Holdings Inc., the parent company of All Nippon Airways Co., was initially expecting the delivery of the plane in the second quarter of 2017.

“The first flight and the subsequent flight tests have confirmed the basic characteristics to be satisfactory,” the statement from Mitsubishi said Thursday. However the company added, without elaborating much, that “several issues” had to be dealt with the jet’s development due to which the schedule was changed. The statement also added that the company will try to increase the precision in the schedule.

Mitsubishi Aircraft, which conducted test flights last month, is developing the country’s first-ever passenger jet in 50 years, while the plane will be the first one manufactured by the company, Bloomberg reported. Senior Executive Vice President Nobuo Kishi said during a press conference Thursday in Nagoya prefecture that the company will strengthen the plane’s frame and will upgrade its software.

“Our initial expectations were too optimistic,” Kishi said, according to Bloomberg, adding: “We found that things that we expected to take 10 days to do took 15 days. Tests that we thought we could do 20 times took 40.”

Hiromichi Morimoto, President of Mitsubishi Aircraft, reportedly said during the conference that the company was concerned if the plane would pass certification tests, which would determine if the plane can withstand 150 percent of normal use. The concern comes despite the plane’s frame passing strength tests for normal use.

“This is Japan’s first new passenger plane in 50 years and on paper everything looked fine,” Morimoto said, according to Bloomberg, adding: “However, when we built it we found places to improve that we hadn’t banked on.”

Japan had suspended manufacturing planes after the country’s defeat in the Second World War, and Mitsubishi Aircraft now wants to get back to making planes, 70 years later, the Associated Press reported.

The company has received 407 orders, including options and purchase rights, for two types of planes that seat 78 and 92 passengers. The planes are believed to be using 20 percent less fuel, due to its aerodynamic design, than the aircrafts in its class, CNN reported last month. The company also believes that it can win a 20 percent of the global market share for single-aisle, regional passenger jets.

“ANA needs to take the first aircraft as soon as possible, but they also want to make sure the quality is 100 percent,” Masao Ueno, a director at AlixPartners consulting firm, said, according to Bloomberg, talking about the impact the delay could have. He added: “Japanese companies are serious about quality, and so the MRJ is being delayed to make sure it’s 100 percent ready.”