Bush again played down Moscow's concerns about the missile shield, saying: "This system is not aimed at Russia. I will continue to work with President [Vladimir] Putin and give him those assurances as well."
Washington wants to put base 10 'interceptor' missiles in Poland and a radar station in the Czech Republic as part of the system.
Polish concerns
The US and the Czech Republic are close to finalising an agreement, but the Polish government has taken a tougher stance in negotiations and many details remain to be settled.
Tusk said Poland was ready to co-operate on missile defence as part of a security effort that would include the upgrading of Polish forces.
"Before my watch is over, we will have come up with a modernisation plan that is concrete and tangible," Bush said, referring to the end of his period in office in January.
Polish officials have said they want to acquire air defences against short and medium-range missiles.
Negotiators have asked for Patriot 3 or THAAD missiles and have identified 17 areas of the Polish military that the US could help modernise.
The interceptors for the planned US shield are for protection against long-range missiles.
The Polish government says that the security backing is necessary because Russia has threatened to target Poland with nuclear missiles if it allows the interceptors to be based there.