The Federal Aviation Administration (FAA) said Boeing discovered the cracks “while conducting modifications on a heavily used aircraft”.
Similar cracks were found in a small number of additional planes, the agency said.
The 737 Max, which was grounded worldwide in March following two crashes, is not affected by the checks.
Boeing said 737 NG operators have been contacted, but they have not reported any problems.
The US manufacturer has not said how many planes would be subject to inspections.
“Over the coming days, we will work closely with our customers to implement a recommended inspection plan for certain airplanes in the fleet,” the company told the BBC in a statement.
“This issue does not affect any 737 Max airplanes or the P-8 Poseidon.”
The BBC has asked Boeing for more details about the cracks, and where they are located, but there has been no response so far.
Models of the 737 NG include 737-700, 737-800, and 737-900. Aeroflot, American Airlines, Ryanair and United Airlines are among the airlines listed as operators of the 737 NG model on Boeing’s website.
In its statement, the FAA said it would “instruct operators to conduct specific inspections, make any necessary repairs and to report their findings to the agency immediately”.
Boeing ground its entire global fleet of 737 Max aircraft in March this year following two fatal plane crashes.
The 737 Max has not flown commercially since an Ethiopian Airlines aircraft crashed shortly after take-off on 10 March, killing 157.
It followed a Lion Air crash on 28 October last year which killed 189. The FAA must decide if the plane is safe.