(Adds analyst comment, share performance, details of deliveries and backlog)

SAO PAULO, Oct 15 (Reuters) - Brazil's Embraer SA , the world's third largest commercial planemaker, reported its strongest order backlog ever on Wednesday after booking a contract for its new military cargo jet and another big order for the U.S. market.

Embraer's backlog of firm orders, a measure of expected revenue, rose to a record high $22.1 billion at the end of September from $18.1 billion at the end of June.

The latest order book reflects a $3.26 billion deal to supply 28 of its new KC-390 cargo jets to the Brazilian Air Force. Embraer plans to unveil the new military aircraft next week and begin test flights by the end of the year.

Embraer's biggest commercial client, regional jet operator Republic Airways Holdings Inc, also signed a firm order in September for 50 more E-175 aircraft to be flown in the United Airlines network.

The planemaker reported third-quarter aircraft deliveries on Wednesday, showing flat sales of its regional E-Jets and a drop in private jet demand.

Embraer delivered 19 commercial planes from July to September, in line with the same quarter of 2013. Executive aircraft deliveries fell to 15 jets in the quarter from 25 jets a year earlier.

"Despite the weaker-than-anticipated deliveries in the third quarter, Embraer remains on track to meet full-year delivery guidance," RBC aerospace analyst Walter Spracklin wrote in a note to clients. "We believe the launch of the all new Legacy 500 has resulted in both a production and delivery push in executive jets into the fourth quarter."

Shares of Embraer shed 2.3 percent in Wednesday trading, in line with a decline of Brazil's benchmark Bovespa stock index .

(Reporting by Brad Haynes; Additional reporting by Silvio Cascione in Brasilia and Susan Taylor in Toronto; Editing by Jonathan Oatis)