Under the agreement, the companies will share specific technical knowledge and jointly evaluate customer markets for the medium-lift aircraft.
The companies said the accord signed Monday broadens an earlier agreement.
"Boeing has extensive experience in military transport and air refueling aircraft, as well as deep knowledge of potential markets for the KC-390, especially those which were not considered in our original marketing plan," said Luiz Carlos Aguiar, president and chief executive officer of Embraer Defesa e Seguranca.
"This agreement will strengthen the KC-390's prominent position in the global military transport market."
"Embraer is a leading global innovator and we both understand the value of working in partnership to provide high-quality, affordable customer solutions," said Dennis Muilenburg, president and chief executive officer of Boeing Defense, Space and Security.
"This collaboration matches Boeing's proven excellence in military transport with Embraer's KC-390 accomplishments to further advance this highly capable and efficient medium-lift aircraft."
Embraer is developing the KC-390 for the Brazilian air force.
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'