PBM-5 Mariner’s had a known history of fuel leaks in the wings and a number had blown up mid-air in the past before this.
Any reason the PBM-5 had a crew of 13 that day? Wikipedia says it has a normal crew of 7.
Remember reading somewhere that the PBM took off at 80 knots, landed at 80 knots, and stalled at 80 knots.