* C919 first flew on May 5 after years of delays
* Symbol of China's ambitions to compete with Airbus, Boeing
* COMAC says plane took off at 7:22 a.m., landed at 10:08 a.m.
* Questions about its duration and 5-mth gap between test flights (Updates to add that plane has completed 2nd test flight)
SHANGHAI, Sept 28 (Reuters) - China's domestically developed C919 passenger jet completed its second test flight on Thursday, the jet's maker said, but there were questions about its duration and on the near five-month gap between the two test flights.
The narrow-body aircraft, which will compete with Boeing's 737 and the Airbus A320, is a symbol of China's ambitions to muscle into a global jet market estimated to be worth $2 trillion over the next 20 years. It first flew on May 5 after numerous delays.
"The plane has made a smooth return," a spokesman for its manufacturer, Commercial Aircraft Corp of China (COMAC) , told Reuters.
COMAC said the plane flew up to 10,000 feet after taking off from Shanghai's Pudong Airport at 07:22 a.m. (2322GMT). It landed at 10:08 a.m. COMAC said, a flight time of 166 minutes. In comparison, its maiden flight was 80-minutes long.
"The interval between the C919's first two flights is extraordinary," said Bradley Perrett, a veteran China watcher and reporter at Aviation Week in an article on Wednesday.
"The conclusion must be that COMAC was not really ready for flight testing in May... A common view is that the C919 was put into the air so early for strictly political reasons, although there is no suggestion that doing so was unsafe," he said.
The Mitsubishi Regional Jet, Japan's first passenger aircraft, took its second flight eight days after it first flew in 2015 while the gap between the first and second flight for the Airbus A350 was five days, Perrett said.
A report published earlier on Thursday by state-backed news website ThePaper.cn said that the C919 was expected to fly between 07:20 a.m. and return at about 11 a.m. COMAC did not immediately reply to questions on whether the flight was shorter than expected.
Strongly backed by Beijing, the C919 has so far announced orders for 730 planes from 27 customers, many of which are Chinese leasing companies.
Last week, Agricultural Bank of China (ABC) Financial Leasing Co Ltd became the first customer to make a second order for the C919 jet. (Reporting by Brenda Goh; Editing by Stephen Coates and Muralikumar Anantharaman)