Investors looking for answers from Boeing first-quarter earnings on Wednesday were left with one big question: When will 737 Max deliveries resume?

The first-quarter results on both the top and bottom line met Wall Street expectations, and the 15% decline in free cash flow was not a surprise, given the 737 Max delivery suspension in mid-March — the plane, Boeing's best-selling plane ever — is estimated by Goldman Sachs to represent 33% of Boeing revenue in the next five years.

It's the importance of the plane — whose software is suspected in two deadly crashes — to Boeing's outlook that will keep investors from being able to have full confidence.

The company's presentation to shareholders noted the commercial airplane business had $1 billion in increased costs due to the 737 production line.

"The 737 Max is the cash-flow generator, spewing off billions in cash. ... If [the company] is not sure when deliveries will resume, there is no way to have confidence Boeing will earn X amount of dollars in 2019," said CNBC auto and airline industry reporter Phil LeBeau.

Boeing CEO Dennis Muilenburg said on Wednesday that he has full confidence in the 737 Max.