The M15 was to replace the M13 and M14 as they weren't meeting expectations.2
The M15 was made from a M3 Half Track chassis. It had the rear compartment removed and the M42 multiple gun mount was mounted on the flat bed.2 The guns could rotate a full 360°.2
The 37 mm could fire 40 rounds per minute and the MGs 500 rounds per minute.2
The M15 had a 37 mm M3E1 and two .50 cal MGs in a combined mount.
37 mm gun had ceiling of 6,200 yards, range of 8,875 yards. The .50 cal MGs had range of 7,200 yards.